Hi Redblanket:
Before you buy anything, I'd suggest you TRY some gear BEFORE you buy. That way you can get a "feel" for the differences.
The obvious leaning here is plates BUT plates can be harder on the body compared to the Transpac if shore (or beach) diving is a LOT of what you do (especially if there's a LOT of walking) or if you have to stay out of the water when wearing your kit. I have dived BOTH the Transpac and a plate system, the plate is a better choice for ease of underwater use (IMO), but that Transpac system is VERY adaptable and DARN comfortable. I use an OMS IQ pac for singles diving and a plate for doubles or singles when I use stages. I use a modified OMS "comfort harness" for the plate which my shoulders appreciate much more when walking with 150+ LBS of gear. Diverite and OMS make very similar products. The DSS plate system and harness is also an excellent choice.
As far as the regulator choice goes, the Apeks recommendations here are very good. It's certainly an excellent choice. I'm a BIG proponent of Poseidon regs, so I'll give you an alternative view. For cold water use, nothing is better than Poseidon. I used a Cyklon 300 for years and all I ever did was rinse it out after use. It NEVER froze up, NEVER iced. When I decided to service it one day and pulled it apart at the shop (probably 5 years and 500+ dives after I got it), the inside of the first stage looked perfect and the valve seat had ZERO wear. The reason for that is because Poseidon is BUILT to a 5000 PSI working standard which not all other regulators match. This means using the regs at lower pressures results in almost no wear. Servicing the regs can be a little trickier, but boxing them and sending them to a service center will probably be cost comparable to local servicing. The Poseidon warranty is also unique. The original warranty is for 2 years, but it automatically extends an additional 2 years each time the reg is serviced by an authorized technician for as long as you own the regulator. There is also no "local LDS" requirement with the warranty.
Poseidon recently changed their "set-up" for North American market X-Stream regulators, they now match the Dive 90 first stage and the Deep second stage. This allows for 5 LP and 2 HP ports on a swivel style first stage, compared to the previous "cylinder" design (click
here for specs). A note on the specs. Although the spec says "air" for the gas, as built the X-Stream is good for up to 50/50 EAN. Poseidon uses a "different" standard for "O2 clean," their "normal" (not O2 cleaned) standard matchs or exceeds most other "O2 clean for EAN" standards used by American companies. I can't give a direct recommendation for the X-Stream other than to say that on a recent "test" dive it performed FLAWLESS and was VERY comfortable. I just ordered 2 of them so I'll post my experience in 6 months.
One thing I can tell you about the Poseidon second stage design. Most people either LOVE it or HATE it. There's no "in between." Test dive BEFORE you buy!
Good Luck!