So, I have the following amp:

http://www.sonicelectronix.com/item_...-MRV-F450.html

I am thinking about getting:

http://www.sonicelectronix.com/item_...pa-100.9W.html

or

http://www.sonicelectronix.com/item_...0GTi-MkII.html

I want to stay with free-air subs cuz I don't wanna sacrifice the little space we have in the trunk. Which is the better bang for the buck that would fit best with my amp?

 

Infinity. The JBL looks to be too big of a driver for you amp. Honestly, it seems to me that your sub channel on your amp is way too underpowered to make either of those subs sound good. If you have to pick one take the infinity as you'll be closer to the power it needs. Honestly its better to give the sub a bit more power than its recommended RMS. Those 10"ers can be slow and muddy if you don't supply them enough. I'm not an expert, but that's what my experience has shown.