Because the silicone is very stiff, I'd recommend going with one that didn't have too many curves/projections unless you know you like that. You don't specify whether you're going to be using the dildo anally or vaginally, but here's my story so far:

I'm a man and I thought I'd try out the F.F.E. 7" G-spot. G-spot dildos are often good for prostate work as well. I wasn't too worried about the bulge at the base, as I can take a pretty big toy and when used in a harness most of the bulge would stay outside in the space between us. So far I've only tried it by hand once, but the extremely stiff silicone and the 1-7/8" projection of the curved G-spot stimulator combined to make the experience not quite as perfect as I expected. Instead of having pleasurable prostate sensations adding to the experience it turned into more of a "milking" experience, where the heavy pressure on the prostate and seminal vessicles pressed out ejaculate, but the pressure was a bit too much for comfort.

The 6" doesn't curve quite as much, but the tip still appears to project about an inch and a half. It would be easier to insert because the tip is narrower by 1/2" (Diameter), and might be a bit more flexible. The 4.5" are probably a bit short for good pegging, don't know about harness-> vaginal use.

The suction cup does seem to work pretty well.