Be sure of the features you value most.
The slim, for example, is said to run a little bit quieter and cooler than the ordinary models. However, we've actually found that it tends to run really, really hot - though it is certainly quiet. The newer ordinary ones have fairly good and quiet fans and processors as well, and they may well be cheaper at this point than the new slim console. But the slim console has wireless networking (which can be quite iffy, but is nice when it works), whereas the older ones have to be hardwired in (more reliable, but often highly inconvenient depending on where you want to keep them) unless you buy the wireless adapter for them. The older models have tactile buttons; the slim has touch-buttons and makes a chiming noise when they are "pressed" that we haven't yet been able to turn off completely. The older controllers have a nicer tactile interface, but other than the one that comes with the console, you get to decide which to buy for yourself, so that may mean nothing to you.
However, consider whether or not you actually want any of those games that come with the bundles - that will depend on your gaming style (for instance, I highly recommend ODST, but you may be more of a fantasy/RPG gamer, or prefer something totally different). That might also be a factor in your decision.
And, finally, check out what WalMart has to offer in the Xbox 360 department. I found them to have quite good deals almost all of the time, especially for bundles that give you freedom (such as ones that allow you to include a charge'n'play kit, or ones that include games but allow you to choose which game you get with it). Side note: the charge'n'play kit is totally worth it and I recommend you get one, and get the corresponding battery pack for each controller you own (you're not likely to need more than one cable, but each controller needs a battery pack). It's much, much better than swapping out a bunch of AAs every few weeks for the rest of your console's life.