I like size based inventory, weight based inventory, and a combination of the two as well. I don't mind to much systems that use neither of them, and either goes for a item amount cap, or unlimited amount... but if I got to choice it would be size+weight.
But no matter what kind of inventory system is used, I will still do everything I can to make sure I can take as much loot with me at any given time, and I WILL go back for the rest if it is possible... and even if we are only talking about a handful of cheap stuff. XD
Anyway... so far, the system I have liked the most is in a series of Russian games where all of them goes by a lot of different names... but Brigade E[incert number 5, 6, 7, or 8, or maybe even 9] is a name that all the game to one extent or another all share, either officially or inoffiially, only the number changes obviously. Although... not all of the games are easy to find under the "Brigade E" name either... 5-7 is, but after that it's not as easy, and I do not remember at this time what their actual name was. Some of the games do not even officially have English versions (especially the 7+ ones), but do have unofficial mod translations to English. Not sure if they are easy to get a hold of though.
It is a somewhat complex system that uses a combination between weight based (where the effect of carrying to much gradually goes up, so no decrease from 0 right up to 50% or something by reaching a "limit" or something), and size based (numbers of squares) where the size of item will matter a lot if it can fit in certain areas or not, with some areas not able to take certain items at all (Equipment slots can only take one kind of item type, where as most Item/Carry slots can take most kinds with some exceptions), and a few slots where size of item does not matter at all. No item that gives you more space can be put inside another thing that also gives more space. Only the Belt+Pouches combo has something like that. Your hands (only one big slot, so no dual wielding) can carry anything. One of the shoulders can only carry weapons that are submachineguns and larget, the other shoulder is the same but can carry a set of armor instead (does not give any defence though), and your Neck is the same as the first shoulder but can also carry a Belt instead (which can have items in it, which can only be accessed by putting it in the Belt slot). Also, most of your inventory is not there from the start either, you need to equip certain things first to get more space to use (Vest, Belt, Backpack, and 0-2 Pouches which can only be put on your Belt if you have one).
Basicly, it is the most realistic weight+size based inventory system I have ever seen so far.
Not to mention that all of those games are some of the most realistic, realtime with pause, timebased (everything you do takes X amount of time to do, where tenth or even hundreds of a second can make a huge difference), teambased, tactical combat simulation, RPG, game, that I have ever played actually.
They go so far as to take into account things like level of adrenalin, injuries, how much weight is carried, what kind of weapon the character is using, being shot at, being hit (with or without actual damage), suppression, etc, into how well your character is able to perform during combat. Adrenalin for example: if you characters adrenalin goes up then your characters "to hit" chance goes down, BUT your characters speed to shot their weapon becomes quicker. Reasons why adrenalin cango up in the game varies, if they see an enemy it can start moving, go around a corner and find loads of enemies infront of you and it might shoot through the roof, getting fired upon, near misses, getting hit, etc, will make your adrenalin going in the game. If your character is wearing a backpack, they are unable to roll when being prone. Your character can bleed to death, get temporarily blinded, temporarily deafened (yes hearing does have an actual effect in the game), get knocked uncontious, outright killed with a single shot if unlucky, and probably some other things I am forgeting at this moment.
UFO/XCOM, old Fallout games, Balders Gate, Neverwinter Nights, and most other other such games, turn-based or otherwise, are simple compared to these games really. So if a person think any of the games I mentioned are way to complicated for them, or to detailed and time consuming and to micromanagy... then the Brigade E5-8/9 is most certainly not for you. XD
I would recommend E5 and E6 though. You can find both on Steam... Search for "Brigade E5", and E6 should show up as well (it's usually recognized by it having 7.62 in the title)... and there are some pretty comprehensible and good mods available for both of them online (Blue Sun Mod for E6 for example... do not recall the mod names for E5).