I am not much into RPG games but I think it is also related to average number of items.

For example, I had found Starbound's grid based inventory quite confusing especially because you could easily put same item to several cells.

while I think SkyUI does a good job with listing

Grid based.
Pros. Familiar, easier to find items visually?, can implement different sized items for some inv management games.
Cons. Micro management of inventory can detract from the game for some.

List based.
Pros. Removes the need for management. Can manage different sized items with 'size' and capacity variables rather than playing Tetris.
Cons, harder to quickly glimse items.

You could display them all at once in the field of view and limit size by weight value.

That way, all items are visible, and items have different mass and you have a maximal capacity.

I always wondered why volume somehow made more sense than mass in rpgs...

I suspect this largely has to do with how many unique items you're going to be carrying around at once. The more limited your space the better a grid works. The more "bag of holding" your character is the better a list works. I believe it's basically as simple as that :)

if you are going to limit something, really limit it

The one utility grid based inventory can have that lists can't, is the "inventory tetris", described before. Item size defined not just by a number of spaces, but by their dimensions.

You can have effectively the same system in a list-based inventory, representing the dimensions with numbers (e.g., height: 1, width: 2); the items would be auto-sorted on a grid under the hood with no visual representation of the grid ever given to the player, except for numbers indicating the remaining space. The usability of that is questionable (someone somewhere can probably come up with a game where that would be cool), but the utility is certainly there. Even manual sorting could be done that way.

Understanding the distinction between utility and usability is important. Once you know exactly the core tasks you want your system to perform, you can build your UI around it; that's designing with vision. If you don't know what tasks you want your system to perform, you don't know what you're building a UI for. The information AlecS provided is not enough to make any real recommendations for his game, and it would need to go through user testing either way.

First goes the type of the game, number of characters, theme, etc :) That's without doubt (if you are making a jRPG where you control a 8 character's party there is no brainer to use the list). That's the most important factor.

Now, personal stuff :) I feel the grid based is outdated... I mean, what's the fun in playing a puzzle if you are an RPG fan? Is it sooo fun to sort out hundreds of skins, rags and fit them into slots? With drasticly increased age of players nowadays, and therefore less time, I think we should aim to compact the gameplay and remove the unnecessary grind and boredom/tediousness. So, I vote for lists.

But then, a list and a list is not the same :) How about weight limit? Is the list an endless bag of holding? Or just a list of items where weight is the real bottleneck? Do you have a lot of small items that weight almost nothing or big ones primarily? How many items you can collect and carry during the game (what's the intention)?

An interesting twist is it to make "a list with 25 positions only" (one game used it, I do not remember the name).

I think DanglinBob has a good point.

If we Look at the Tetris grid, Icon grid and list inventories; we can see there is a clear difference in the items hoarded.

Consider these three items: Sword 5 dmg and 10 Gold / A Broom 2 dmg and 2 Gold / A Cup 1 dmg and 1 Gold

As far as gold is concerned the value of each item is 1 Sword = 5 Brooms = 10 Cups

Now with a list inventory a new value will be added to prevent players from looting any thing that isn't tied down, the new value is weight. The Sword 3 lbs / Broom 1.5 lbs / cup 0.1 lbs.

30 Cups > 1 Sword > 2 Brooms

In Icon grid any item takes one slot, often these items have a stack value to control there worth. Sword 4 Stack / Broom 4 Stack /Cup 9 Stack.

4 Swords > 4 Brooms > 9 Cups

For the Tetris grid each item consists of small squares (X,Y). Sword 3*2 / Broom 4*1 /Cup 1*1

1 Sword and 2 Cups > 2 Brooms

These different inventories allow the developer to place value where it is important, also to tweak the values of each item.

I agree with Nyaanyaa, all of these inventories can be tweaked to have the exact same values and to function the same mechanically. The important difference between them is how the mechanic is experienced, from the players viewpoint.

I also want to point out that even if all of these systems can be used in exactly the same way, most developers don't use them the same. The real world has a influence on the value of items, even in games, these small changes cause the inventories to differ drastically.

Now, personal stuff I feel the grid based is outdated... I mean, what's the fun in playing a puzzle if you are an RPG fan? Is it sooo fun to sort out hundreds of skins, rags and fit them into slots? With drasticly increased age of players nowadays, and therefore less time, I think we should aim to compact the gameplay and remove the unnecessary grind and boredom/tediousness. So, I vote for lists.

There's definitely an older version of grid based that seems outdated, like the old X-Com and old Diablo games, where some items take up 2x3 spaces or more in the inventory. That kind of inventory management feels more like an unnecessary hassle most of the time. However, there are new games with grid based inventory too, like Diablo 3 (all weapons and armor are 1x2, with some small items like rings and crafting materials that are 1x1). WoW and Don't Starve have grid based inventory where everything takes up 1 grid space, with some items being stackable.

I prefer simple grid based inventories. Instead of limiting items by weight, they limit what you can hold just by a number of spaces. It's less realistic, but generally I don't want to deal with my inventory more than that. I also like the more visual representation of what I'm carrying to a list. However, I guess this feels more clunky once you have so much stuff that you need several packs, and everything gets spread around unless you organize it yourself.

Bethesda comes to mind for list based inventory, using weight to limit what you can carry. With weight in Bethesda games I always end up getting too involved with the math, making sure I'm carrying the best value for the weight. I don't really enjoy it, but I get caught up in it nonetheless.