In terms of hardware, the 2 are similar but the Evo has an 8MP camera, compared to thr 5mp on HD2. Also a front facing camera, hdmi output and kickstand. The most important thing to realize is that the HD2's Windows mobile 6.5 is a dying platform, although u might be able to flash windows mobile 7 on it, it wont be official and might give u trouble later on as that platform progresses.
I personally haven't had any problems with sprint service. A great perk of being a sprint customer is when u get the EVO, you will become a "Sprint Premiere" customer, as it'll require u to be on a plan of 69.99 or more per month, so you will be eligible for an upgrade next year around the same time.