Walrus and Hoops are dead on here, the extra years production years you would get out of Wallace are obviously preferable if all things moneywise were considered equal. If you ask me, if the braintrust is actually interested in bringing in a top FA WR, which is still up for debate, money is going to be the determining factor. They are both going to cost a pretty penny to get to come south, but Wallace is going to cost a chunk of change more.
I can't speak for actual on field intelligence comparisons because to be honest Wallace is still essentially in the growing phase as far as expanding his all around game is considered. But as far as where they stand right now, Greg Jennings has got it all. Sure hands, willing blocker (even though GB doesn't quite rely on their running game more than to keep the opposition honest), and can run every route in the playbook. On the other hand, if you believe Wallace will continue to grow as a receiver and not plateau from a straight line burner a la Desean Jackson, which I am more than skeptical about, then hell, he is worth every goddamn penny you got. He is on the cusp of becoming a truly elite receiver in this game especially with his gamebreaking speed and with the continued rule changes to allow a receiver to basically go untouched in the secondary.
The bottom line is that I have a feeling the money gap between the two is going to be huge. I wouldn't be disappointed if i woke up in March and either of these guys are passing on the Breaking News ticker as the newest Miami Dolphin. My biggest concern is that you will be paying for what Mike Wallace COULD BE in a few years down the road rather than what he might actually be worth then, while you know what you get out of Greg Jennings and it is fantastic. People are throwing out how injury prone Greg Jennings is and its a whole lot of smoke. This year he basically had a hernia and kept coming back to early, give him an offseason and maybe the only side effect is that his asking price is slightly lower.
Whatever the case, you add somebody of Wallace or Jennings talent level to our offense, and even the blindest of Dolphins fans will realize what this offense's ceiling really is. Give your franchise quarterback somebody who can find the end zone to throw to, and enjoy the ride.