GTAV
pros: fantastic and good lengthed story, lot's of things to do, slight humor, slightly less-unrealistic features i.e. driving and cover systems, massive multiplayer, online racing and group shinanigans are really fun
cons: after the campaign is finished those without online may find its lasting appeal shortened (cops are much more dangerous than sr4's), the multiplayer has kinks, some missions require coordination that some players will ignore and others will try to dominate on, forms of deathmatch offer quick cash and shooting skill but not alot of fun or excitement, loooong grinding for cash or respect can dullen out parts of online, and because it is an open world multiplayer there is the chance an ass whole may ruin your fun, and no PC version yet which means modding gta5 is not even a question at this point
SR4
pros: great humor and story, memorable characters, more customization with the protagonist and vehicles, combinations of wacky weapons and superpowers makes for fun, Co-op powers activate! better to manage who your playing with and usually done with a friend , and some modding with promise of better with tools
cons: compared to its great predecessor SR2 some customization still feels downgraded, the story without partaking in side activities is short by most standards, after the initial grinding it is very easy to become rich and overpowered, and minor bugs seem to dot the game
Sidenote: I can't believe I'm gonna say this but I find SR4 alot more family friendly than GTAV, if I had a kid I would feel a helluva lot more comfortable giving him a copy of SR4. Both have sexual themes but Saints is goofy sexual, hitting things with an alien dildo-bat is funny and everything else is either censored or a big joke, in some cases both. Bad language in Saints is pretty sparse if not lesser the amount kids say to each other these days. GTA... well let's see, interactive torture part, interactive prostitutes, lot's of subjects you don't want to mess with, just about the only line Rockstar doesn't dance on is making porno, although gta online is much cleaner I might add but riddled with annoying 10 year ironically
On the other hand you may want to get GTA because that's what your buddies are playing or Saints because it may become the next big modifiable game (yeah that's right, move the fuck over Skyrim)
As for SR5, I think the game companies really need to take a hard look at what they are doing becuase in my opinion most of the well known games have had some shitty sequals recently (army of two, 2 amazing games and then that shitty 3rd), with few exceptions. I think Saint's Row is too big a title to just let die but my fear is it will also repeat its gameplay style. The other two scenarios is a major rebrand or getting put on the back burner for a long time
Edit: get gamefly, rent and play more games for way cheaper than buying and keeping forever, plus discounts on pc games