Both games are fantastic.
But they are a little different.
There you have it... brief-style. In case it isn't obvious, I love both games, and could never choose between the two.
If you still aren't sure, figure out if you want more outside long range type missions, or more close quarters type missions. That may help you decide.
1. "You have a targeting cursor only" - True... which means you see more in GR but don't get to see the cool modelling. It's neither good or bad; it's the smallest factor (for me). I'd say the targetting reticule is the bigger difference; GR is less forgiving than RS3 and RS3:BA as it requires you to AIM.
2. Rather than focusing on sniper rifles, focus on shotguns. If you take a shotty in GR (I don't recall if there IS one) you'd die. A lot. The range of the sub, assault and sniper class rifles are (generally) required to get the job done.
3. "Ghost Recon uses an older engine, and it shows..." VERY MUCH so. Raven Shield looks better. Functionally, it's not as vast as the GR series and the graphics are more immersive in GR than Raven Shield. In RS3; basically you're shooting people around corners, and the triggers are really obvious.
4. "Ghost Recon has some pretty kick-ass atmosphere." I agree. GR brings surround sound and shows you how to use it. I didn't find RS3 to be much help with it, although the voice comm was fantastic. The thing is, GR lets you move your troops via a map which is much nicer (overall)
5. " Black Arrow has more options for multiplayer " And bullets miss people, and it's just an expansion pack for 40US. I have it; I'm gonna be trading it in for Fable. I've already beat the crap outta RS3 (original), so it's been traded. I don't own any GR right now, Island Thunder was the same thing as RS3:BA (should have had more; added a ton of new weapons and *way* more maps to be truly enjoyable).
I'd recommend getting either RS3 or RS3:BA used (they're practically the same game) or either GR or GR:IT used. Save your "new" title for GR2 this fall. It apparently goes 3rd person; but it does mean you have a better peripheral vision. Plus there's a 1st person mode for the better gamers.
and *that* is my humble opinion. "Take my advice; I'm not using it anyways"