This is a good question that deserves answer longer than one or two lines. Sorry if this seems long but it may help.
This will be long----
First we have to clear up something,
a BCD of this type consists of 2 systems:
1) Harness
2) Wings
Next, I really believe that a back plate style BC is the way to train. When I say back plate style I mean plate and harness system. I know they look uncomfortable, and they are way more expensive then they need to be. But one has to look past all that and see the value and the performance that they offer. First off they are not comfortable while youre on the surface. That is true, however what BC is, and they are no designed to be worn on the surface. They are the most secure platform for any type of diving. Once in the water the cylinder will not move, which is a good thing. The Transpac II seems more refined; it may be, but it is also overly engineered (Maybe its the way to go if you are diving side mount --but as I do not and most do not for that matter why use it). Why do they make it the way they do ---because the manufacture needs to sell them---they need to make them comfortable on the surface because that is where most people try them on and then buy them based on the feel of the BC on the surface. They need to look cool and have lots of bells and whistles! Not saying DiveRite is makes a bad product. In fact I think Dr has better Customer Service then anyone in th industary ---talk to Dyson--he will spend lots of time answering any questions.
Most people that buy BPs already know the value of them, they dont need to look cool or be comfortable on the surface. Most new divers do not even know about BPs because they are not exposed to them until much later in the training process
Back plates:
Now to me it doesnt matter which manufacture you buy from--- Diverite and Halcyon both make fine back plates and you can even buy a harness from them----although you can easily make one for 20 bucks.
Wings (bladder):
This is something that is really important to some and not so important to others. Wings (Diverite Classic or RecWing, for which I have both) are really nicehowever they have drawbacks. The classic will create more drag when diving with single cylinders because it is really made for double manifold cylinder diving (diving with doubles). The RecWing is more streamlined and better suited for single cylinder diving. However it has a bungee that is really not necessary. At any rate it can be taken off, but you would always have the loops sticking out of the bladder. The Halcyon setup is straight foreword, no nonsense, company. Halcyon menses no words when they say no BCD can do it all. They make the Pioneer and the Eclipse for single tank diving. They are both fine wings that are very low in drag, I prefer the Eclipse due to the fact that I can change out or fix the bladder. Something that will save you 300 bucks if you were to ever get a hole in the bladder. OMS is now making an Eclipse type knockoff; it actually looks pretty well made, but it does look as though OMS is playing catch up with Halcyon. For double tank diving Halcyon makes the Explorer, its basically the same as the Diverite Classic Wing.
Once you get to the point where you are ready to go onto Doubles all you will have to buy (BC wise) is a wing capable of lifting that much weight: Diverite Classic, or Halcyon Explorer. If you go with the Transpac II you will eventually get to the point (which I did) where you will outgrow the BC. You will never outgrow the Back Plate and Harness. Also as long as you stick with using the one-piece harness you will always be using the same set-up basically. You wont have to get used to a new type system, which may help you concentrate on the details of using doubles.
So to answer your question I would go with the Back plate design with a one-piece harness (3 or 4 D rings and a SS Buckle), a low profile wing such as Halcyon Eclipse or Pioneer. This system will be the simplest and most adaptable BCD system you can buy.
