I'm looking at these 3 phones and here's my impressions based on checking out the phones in a store and reading reviews and forum posts (I don't have any of these phones yet so take these w a grain of salt):
Speed: GPU speed is Fascinate>X>DINC with Fascinate holding a fairly commanding lead. CPU speed is the same order although it's a much tighter grouping in this area. However Sense UI on the DINC is pretty polished so many folks feel that the DINC is actually faster/smoother from a UI standpoint in daily use. Surfing the web or playing games... the Fascinate and the X beat the DINC.
Battery life: X>Fascinate>DINC. The X also has an extended battery that adds almost no bulk to the phone. The DINC can use TP2 batteries which are cheap and are 1500Mah instead of the stock 1350Mah battery. There are also extended batteries that come with a rather bulky (some say ugly) back cover. I haven't found much info on extended batteries for the fascinate yet.
Camera/Camcorder: I can't really get a good handle on this. It sounds like the X does not do very well in low light but it does have a slick panarama mode and is otherwise solid. The DINC appears to do pretty well as does the fascinate. The fascinate may do a bit better in low light then the other two. From what I've gathered the fascinate does better from a camcorder standpoint then the DINC and the X but the X does have a 3 mic setup that gives it an edge in sound quality.
Phone: Sounds like the X has very good signal reception and it has a 3 mic setup that gives it very good voice quality and noise cancellation. Downside is that the ringer, earpiece and speakerphone are too quiet (although the sound is good). DINC has good reception although the earpiece sound quality is just ok. Fascinate has ok reception, speakerphone/ringer and earpiece volume are all good.
Screens: The X has a higher resolution screen then either of the other two devices and it is considered to be very good for an lcd screen, especially for text. The DINC is the most pocketable and easiest to use with one hand due to it's size, it has very good contrast ratio, only real downside is that the screen isn't quite as good outdoors. The Fascinate has the best contrast ratio screen and holds up well outdoors. Very good screen for media. it's size gives the phone size a nice middle ground between the X and DINC.
Software: Sense is polished, fast and includes a number of nice looking and useful widgets and applications including leap, T9 dialer, weather widgets, clock widgets, good email client, etc.. TW is good, it includes a couple good widgets, includes a T9 dialer and a handy pulldown notification bar toolbar. Motoblur (not really motoblur) has a decent selection of widgets, many are resizable.
Issues:
-There are concerns about the GPS capabilities on the fascinate.
-The fascinate has bing loaded as the default search provider and it may be difficult to remove (I personally don't see this as a big deal but it seems to be a big issue for a lot of people)
-The X has problems maintaining a connection on an n wifi network, g is fine.
-The X email client has problems with some mail servers, motorola/verizon offers a free license to the touchdown email client (at least I think this is still available)
-The X is pretty tighly locked down, although there are reports that this has been bypassed.
Other: Froyo is available now for the DINC.