mfalco:
The only BCs I've used are rental sherwood silhouette. They were very comfortable and seemed like they would do the job, and they must be durable if they are used as rentals. Other than if I want integrated weight, is there any reason I should purchase a more expensive BC?
Short answer: In order to safely make single tank recreational dives, NO.
As for integrated weight here are some thoughts...
For cold water diving you will end up with more weight than you want to have in your BC. In fact you may have more than it can even handle. From what you say of your built you would probably make it especially with a steel cylinder. The problem is that your BC becomes a heavy pig. The advantage to this is that the weight and buoyancy are in the same item so if you inflate the BC at the surface it holds the weight for you instead of trying to hold you which the weight is hung from. In other words it's less likely to ride up on you. This isn't a big deal and a well fitting BC or BP/W will control this.
Another reason some like an integrated weight BC is that they have no hips and fear loosing their weight belt. Adding suspenders to your belt or getting something like a DUI Weight & Trim harness as much better solutions IMO. I went with the DUI W&T for my drysuit, it also gives me much more range for balancing my trim.
In milder water integrated is convenient. When I dive summer time fresh water at the pond I wear my 5mm fullsuit and just add 2 4 pound weights to my BC, no belt, free and easy. You can see where this may be attractive as you venture south.
With a weight integrated BC you will probably have the ditchable pockets (generally 2 @ 10 pounds each) and 2 non ditchable pockets up on your back,( Generally 2 @ 5 pounds each.
To get my trim right I need two 3 pound weights in my trim pockets. This effectively puts about the same weight over my lungs as a common stainless steel backplate. I then have a 3 pound weight in each of my ditchable pockets. For fresh water I leave these out. I then wear a very comfortable 14 pound (hard) weight belt. Distributed like this with my 7/7 suit I have nice trim and nothing is too heavy to handle.
The point of the last paragraph is that you may not want to stuff all your weight into a weight integrated BC but it does provide some handy placement options. Alternatives include straping trim weights to your tank with an extra cam band, and clip on weights on D rings. Using some of these methods your Silhouette will do fine. I did some of my pool work in that Sherwood model, it was fine.
I know some divers who are diving 7/7mm suits and aluminum 80s who need to have some weight in their BCs just to keep the belt reasonable in weight. I dive exclusively steel cylinders in home water.
As for back inflate, wings and all of the rest.. Let me offer an observation. Virtually any jacket BCD has a fair portion of the air cell on the back side. If you are properly weighted you will not be diving with a huge amount of air in your BCD hence the bubble you do have (or the majority of it) will be on your back and you will hang nicely below it.
I have not tried a BPW but I have tried several back inflate BCDs. Inflated reasonably they did not plant my face and it was easy enough to rock back into a raft mode. What you wear for a wetsuit, where you place your weights and what you dive for a tank all play into this balancing act. As you read posts and comments realize that you must understand that divers configuration and diving to put the comment in perspective.
Now to your question...
Do you own the Silhouette or is this something you have been renting? If you own it and like it then I'd say dive it untill you have other gear in place. If you don't own it then try as many rigs as you can. By the time I finished my cert dives I had used 6-8 different BDS some in the wrong sizes and I realized I just wasn't that fussy. I've been very happy with my Sherwood Avid.
Here's what I saw in the rig:
http://www.scubaboard.com/showpost.php?p=1142032&postcount=2
The bottom line is you want something you are comfortable in. Take your time ask a lot of questions come to understand this crap. I honestly belive that truly understanding your gear choices is far more difficult than becoming certified. There is no one right answer. We are all different as is our diving, budget and passion for this sport. Take your time and make smart choices.
Pete