Depends on how your going to do the switch. If your rich and you don't need to use an update and then go ahead and switch, D2G is a great device. But if your not rich then i would stick with the fascinate and wait for dual core or more of an upgrade (i guess 2.2 is a huge upgrade but you know what i mean, spec wise) One thing that should be noted is that you will feel like the screen is a lot smaller. The moto's have really really wide screens and it makes them feel smaller. My ipod touch screen (3.5in) feels bigger than my D2Gs(3.7) simply because it is much wider.
Hope this helped.