A few pointers:
(1) Don't go with a proprietary system (Vac-U-Lock or similar) to start with - you're limiting your selection of dildos, and very few of them are silicone.
(2) In general, "Jock" style 2-strap harnesses are easier to adjust and a bit more solid. Good midrange ones I know of are the Sportsheets Latigo Leather and Aslan Commando. In the more expensive range I've heard good things secondhand about the SpareParts Joque, Outlaw Leather and Aslan Jaguar models. All of these are O-ring harnesses, and most have interchangeable rings so you can use almost any size dildo (exception: the Joque).
An O-ring harness will allow you to use a wide variety of dildos, the only limitation being that they need to have a base that can "catch" in the O-ring (or be a "strapless double" such as the Vixen Nexus or Fun Factory Share). Look for a flange all around, as the dildo can work it's way out if you're only relying on a realistic's "balls" to keep it in. If your setup will be used anally then do get silicone - much more sanitary and the flex makes it safer during use. If you're going to use it anally I'd say work with fingers and other toys until you can take a 1.5" diameter dildo of at least 5.5" long, so it's stiff enough to be stimulating and long enough that you and your partner can thrust without it coming out. I like the Tantus Curve, but good things are said about the Silk Medium, Mistress, and other ones of that style. Curved dildos are more stimulating and are easier to insert anally, and a head can provide a visual "landmark" as well as a nice bit of added prostate stimulation.
Lastly, but possibly most important, get something that both you and your partner are comfortable with. Something that makes either one of you feel uncomfortable will make the experience less pleasurable. If either of you wants non-realistic, then get a non-realistic even if the other one wants a hyper-realistic 8"-long monster.
Personally, either of the harnesses listed works with the following well: Tantus Captain, Tantus Curve, Vixen Nexus Maximus, Fun Factory Share, Vixen Maverick, Vixen Astro, and others that I'm forgetting. The Tantus Hank tends to work out (not enough top flange), and a dildo with a flimsy base in a large O-ring might do the same, but you can get a smaller O-ring at the hardware store.
