I did the same thing when the 900 was first released. I liked my Arrive, but data speeds on Sprint's 3G network had been getting slower and slower each passing month to the point where I would open up Bing Maps and the basemap tiles wouldn't even load. Not load slowly - they wouldn't load at all. Speed Tests using Bandwidth (RIP) and/or dslreports.com/mspeed would regularly indicate download speeds of 80-200 kbps. Sure, Sprint has unlimited data, but I was only using 200MB per month because I would wait until I got onto a WiFi network to do anything more than text or email. I was thinking about waiting for an LTE Windows Phone on Sprint but when the Lumia 900 was available for pre-order, I was so fed up with Sprint's data speeds that I made the switch.
Now that I'm on AT&T, data speeds are so much better. LTE speeds are insane - the Bandwidth app would indicate speeds as high as 20Mbps - and "4G" HSPA+ regularly gives me 2-4Mbps. Over ten times as fast as Sprint 3G. The trade-off is somewhat flaky coverage and a more expensive bill. Honestly I'm not yet sure if it has been worth it, mainly because my 900 (the first one and the replacement) have not impressed me. Sure, build quality is outstanding, the screen is AMAZING, and it is much snappier than my Arrive in every way, but I've had several issues: the original data issue, a different WiFi issue that I never experienced on my Arrive, and the purple screen issue. Oh, and the worst issue of all: the camera is legitimately bad. It's on par with the camera on my Arrive. These issues aren't dealbreakers but they've left a bad taste in my mouth, especially since Nokia's reliability and camera prowess are two major reasons why I decided to buy the 900. Maybe I was simply misinformed on those topics.
I'm also a little annoyed at how Android has seemed to catch up to WP7 in terms of UI speed while WP7 seems to be standing still. I dunno. The Evo 4G LTE looks really, really impressive in many ways, especially the screen and the camera. Some days I think I should've stuck with Sprint and waited to see if I liked the Evo. Oh well.