To clear the air a bit, Verizon advertises 5-12 Mbps down, 2-5 Mbps up, for their LTE network. Sprint advertises 6-8 Mbps down for LTE, 3-6 Mbps down for WiMAX. T-Mobile advertises around 10 Mbps down for HSPA+.
Of the four networks, T-Mobile's is by far the most loaded...any 3G-enabled T-Mobile phone uses their HSPA+ network in some form or fashion, and every single T-Mobile-branded smartphone uses 3G. Thus, it comes as no surprise that advertised and delivered speeds are comparable in many places on their network, though in others T-Mobile exceeds expectations by 50-100%. This is speaking from experience.
Next comes Sprint WiMAX. It's about as loaded as it's ever going to get; prepaid customers will continue to be added, but Sprint postpaid customers will end up on LTE in relatively short order. On the other hand, WiMAX service is poor in some areas where it's deployed. The upshot of all this is that, when you average the lousy speeds (in some areas) the the stellar speeds (in others, e.g. 12 Mbps down and 1.5 up), you get 3-6 Mbps.
Then comes Verizon LTE. The network doesn't have very many customers on it right now, since the only devices that use the network are ones that came out in the past year and a half, none of which can be had for free from VZW's website. The result: speeds on the network are well above advertised in many cases. I've seen 25 Mbps down, 10 Mbps up on my iPad. It's wonderful, but don't expect speeds to stay that way as Verizon loads more customers onto its network and doesn't add any more capacity beyond the 10x10 LTE Rel 8 carrier they have now. Remember that every single VZW LTE device on the market at this point can only use upper C block 700MHz for LTE, and while 700MHz is great for coverage, adding capacity to the network on such a low frequency will be a more delicate maneuver (surprise, surprise, Verizon wants more spectrum, at higher frequencies, to alleviate this issue).
Finally, you have Sprint LTE (does AT&T post expected speed numbers for its LTE network? If not, it's because 5x5 behaves very differently than 10x10). Sprint's smaller cells (due to higher frequencies) allow it to provide a more consistent experience on LTE than Verizon (6-8 Mbps vs. 5-12) because you're covering a smaller territory (less customers)...with less spectrum (a single 5x5 carrier now...more later). That said, Sprint's network will start off lightning-fast, since as of July they'll only have four high-end phone models (the lowest-end being the Viper) and a couple of mobile broadband devices riding on the network.
Eventually speeds will slow down to the 6-8 Mbps that Sprint is talking about, though my bet is it'll happen more slowly than with Verizon since Sprint will be making incremental upgrades to its network (refarming PCS A-F CDMA to LTE, adding LTE to SMR, upgrading to LTE-A) over the next 18-24 months.
All that said, if I can get a reliable 6-8 Mbps down and 2-3 Mbps up on my Sprint LTE phone from the time LTE launches until the time my contract is up, that's enough for me. If I ned to download something super-fast, I'll have a 30M down, 5M up cable connection at my apartment for that (or whatever TIme Warner Cable lets me get that has higher upload speeds).