More often than not, I have found they're sold as a pack inexpensively? But, to take your question at face value....
I'd have to say EFLC has more potential. The narrative in GTAIV is good, and the gameplay overall is nice (I prefer the actual storyline here), EFLC pushes the boundaries a bit. There are more weapons that are more creative, and additionally there are some more flying vehicle options. The GTAIV campaign is longer though.
Graphics? No difference. They're on the exact same engine. The only thing they did for EFLC was set up a different story and add some new weapons.
Smaller download? Both are roughly 15GB each.
For replayability and messing around, I'd keep EFLC on your hard drive, but there's no reason you couldn't buy them as a pair and remove GTAIV when you finished with it.
As a tip, if you want to avoid the Games for Windows Live crap that the games still heave integrated despite it being a dead platform, you'll need to google for something called "xliveless.dll" A note to the Toms mods, should they take opposition to me saying that: It doesn't crack the game, and the platform it interferes with is no longer in service and vestigial to begin with
TLDR: They're often sold as a pair, and that's how I'd buy them if you can. Particularly on sale. Both games are ~15GB in size (each), so no difference there, and they use the exact same engine and graphics, so again, nothing unique between the two. On the whole, I'd say EFLC has a better longevity, but I'd strongly suggest considering playing through GTAIV first and then uninstalling it if space is an issue to make room for EFLC.
I don't recall whether I just damaged my install or if they are, but they may not even be independently operable anyway. The total size for the steam folders I have for GTAIV and EFLC is 30.5GB.