FWIW, I currently have both the T-Mobile Springboard, and the Plus...in a nutshell,in the 5 days that I've had them:
Plus: better battery life, OS/apps/widgets, lighter weight, processor (sling box video stream/Netflix played smoother), speakers, works on AT&T HSPA+, infra red for A/V remote control, has camera flash
Springboard: better screen resolution, cellular reception (picked up more wifi networks in my neighborhood, but fluctuated 4dBm weaker on my network)
The only real issue I've had is with the Springboard not "displaying" input registers when using web browser...if I press a link, the link may not change color, nor the status bar appear, so it looks like the screen/system is not responding however if you wait a bit, the page eventually loads. Fortunately, Firefox runs much faster on both tablets than on the phones I've used.
Not sure which one I'll keep...waiting for unlock code for Springboard...if it supports AT&T 3G, I'll keep it (remote desktop looks much better), and revisit decision if/when the 7.7 comes out, because if the plus had higher resolution, it would be an easy choice "for me".