I'm looking to buy my first urethane ball but don't know where to start. I'm a 16-17 mph tweener bowling on wood that has very snappy backends. The visionary crow looks interesting but will appreciate the advice from people in the know.

The asyms mentioned will back end harder than the Crow for most part (depending on drilling).  The Crow is strong but super smooth and more forgiving release wise.  Some of those asyms may carry a bit better depending.  Not sure if Crow too strong for wood as only bowl on synthetics but love my Crow and even with my low revs I can use it on THS.  Doesn't carry as well as my Sumo though but can handle more oil.

Depends on how strong of a urethane ball you're looking for. The Black Widow Urethane, Hot Cell, and Shadow Ops will be the strongest of the bunch.

The Crow falls just below them.

After that you're into the area of the Boo-Yah!, True Motion, Pitch Black, Hammer Black Urethane, and Tank Rampage.

And finally you have Combat Tank, Purple Pearl Urethane, and Pitch Blue, although I wouldn't really consider them "weaker" than those above just a slightly different motion.

For what I imagine a tweener to be with 16-17 MPF (which could still be far from what you actually are, lots of factors come into play) I would probably go with something on the higher end. The Shadow Ops looks really, really good. The Crow is also great but I think the former is a bit more versatile overall. If you can even say that about urethane/urethane-like reactions.

Purple Hammer is stronger than the Black Hammer in my opinion. Not entirely sure if the Pitch Blue is stronger than the Black. (Stronger as in it picks up early and noticely stronger downlane.)

Not sold on the strong core asymetrical urethanes.  Urethane is at its best when you can play direct and keep the ball in front of you rather than playing out angle.  Even the strongest core urethanes are going to lose their hitting power quickly when you hook them very much.

The hot cell was the promo ball up at the recent TAT.  Lanes were flying but I didnt see anyone bowl well with it or any urethane ball because there was no way to go direct with them. 

Bottom line, any urethane you pick is going to be very condition specific.  I like the core shell matchup in the Pitch Blue and Pitch Black, but some of the others may be just as good.   

This is the issue. 

If you want to try urethane, you will want to find one that matches up with you going as straight as possible while still generating enough carry to warrant its use.  For some that may be a strong asymmetric while others may want a weaker option.

I personally can't use urethane on my THS.  I have an original Black Phantom, but my launch angles are too steep to use it effectively.

Through years of coaching, the thing I noting most is that bowlers don't keep the ball in front of them and on line consistent enough to see the benefit in using urethane. People think they're throwing it like Chris Barnes in position round of the US Open, but after years of bowling on the house shot they develop a bit of a belly in their ball path. Just look at how many guys say they're playing "right up 10" and in reality its about 14-7 at the breakpoint. The ball going through the heads is so quick that our brain gets tunnel vision to the pins and it's we aren't correctly processing where our ball is going.

The true benefit of using Urethane comes in the ability to get the ball down the lane in a straight line to the target. The slow response time lets you camp out in one spot until you can get the corners out and misses inside are a money shot everytime.

As far as a "proven urethane" I don't think anyone can go wrong with a Pitch Black or Purple Pearl Urethane. The Hot Cell and Black Widow both flare 2-3 times as much as the aforementioned and it's going to start arcing in the midlane a lot sooner. That's not beneficial on a house shot or an easier pattern, especially if you can't feed the ball in a straight line to the "spot". The Pitch series and Urethane Hammers stay much straighter through the heads and midlane before making that soft urethane arc motion. The BTU's from Brunswick both go much longer and act much like the early weak resin balls like the Blue Hot/Red Hot flames.

Just my experience.  I have a Storm Super Natural and recently picked up a Black Widow Urethane.  The BWU is a beast on synthetic, holds very well, doesn't over react, and hits and carries very well. 

The SN goes much longer with a smooth backend, but the carry doesn't seem very good. 

On old wood (my league is old wood) the BWU is basically unusable for me, even on fresh.  It just hooks way too much and is not what I was looking for in league. 

I only throw about 14-15 mph and a stroker. 

Since I bowl on fairly heavy THS on synthetics for most part and being speed dominant and low revs urethane is pretty much for practice only (and my spare ball).  Is a blast to throw and is great for practice because of durability and less oil absorption on your reactives.  Never thrown over about 230 with any of my urethane pieces though.  Just don't have the carry I need.  Tend to be more consistent with them though.

Whenever i get a urethane ball, and ive had a bunch, i almost always take surface up from whatever it comes from the factory, to at least 3000 and almost always end up putting Polish/Secret Sauce especially on the ball. Just seems to work better for me in getting thru the heads all times. Now, it does limit what i can throw on, but for me i really only bowl THS so i usually have a place to play. On the fresh, it takes a game to get a shot so take it for what its worth if you have a game to play with it.