I'm thinking of making a map based on the college I go to (by which I mean parking lots, buildings, campus, etc), but I'm not sure if I should use the tools for Portal 2, Half-Life 2, or L4D2. Anyone have any ideas about this? I kind of have mixed feeling about this.

I've been thinking of doing the same thing for my campus, either for Garry's Mod or L4D2 survival. Been gathering snapshots of the maps posted around campus as reference material ... not sure if I'll have the time to actually produce anything though. School itself kind of dominates my time at the moment.

I'm kind of leaning towards L4D2 because of the nice modern textures that could be used, but I'm still unsure.

as someone who has been mapping since Doom2 (the original DOS based version) let me say this is an "idea" SO OLD that I would be willing to be EVERY mapper has had it. Not that it's necessarily a 'bad' idea. just it's definitely not original. and most people get bored with it before they ever get to anything "cool" (Real world has too much detail, and you can NEVER make it well enough to ever be happy with it, if you're a perfectionist)
Now. With that said: If you're going to REALLY stick with it, and give it the detail and attention it would require to not make it suck lemons then I would go with L4D2 for a few reasons.
Most "real world" structures are not made to be "puzzly" (ie: Portal) or good places to do CTF/CTP/ETC (ie: TF2). So that leaves HL2 or L4D2. Well HL2 is great and all. but it's like eleventy years old AND it's more designed for small "mission" style setups (granted with the UNLIMITED mods out there I guess you could pretty much customize it to whatever you want).... But, In the end... Who DOESN'T want to live out a zombie apocalypse in their own 'home-town'... If for no other reason than to get some practice for when the REAL zombie apocalypse happens in December 2012

I also did a map of my high school for Duke Nukem 3D back in the days... which got popular in our class

halo 4, December 31 2012.
a clear sign of the apocalypse.

I think I'll go with L4D2 tools. It makes more sense to use that.