As far as I know there's just the one 'official' wifi shield, so that kind of narrows down your choices.
Also if you just want to transmit a couple of analog inputs you might want to look into getting a pair of XBees. They are pretty cheap and do point-to-point stuff well, and relatively simply.
If you want to wirelessly transmit data, the two options that I recommend (extensive community use, so you're likely to find working code for both) are Xbee or RN-XV.