Jim,
I have a WBB E7 and also have a 3rd Rail E7 on pre-order for May delivery.
The 3rd Rail engine has a horizontal drive, meaning there's more room inside for the supplied crew cab and a larger speaker.
My WBB E7 was an unpowered unit, I bought a single "A" unit as I didn't want/need a "B" unit. I then bought the power upgrade kit to power it. I had to strip/repaint my WBB E7 to get in in Seaboard colors, the 3rd Rail engine is coming already painted.
I added the ERR Cruise Commander, MRC AC Sounder, and LED headlight, they added an additional $150 or so to the cost. It's still conventional, just has cruise control. The 3rd Rail engine comes with TMCC.
If I had known 3rd Rail was going to be doing a Seaboard E7 I would have not spent the money on the unpowered WBB E7, the power upgrade kit, the ERR Cruise kit, the MRC AC Sounder kit, or the few $$$ to put in LED lighting and to strip/repaint the engine and decal it.
I know my situation was not common, but you can see that to have a WBB engine that "comes close" to the 3rd Rail engine ends up costing close to the same amount of $$$. I'd say MY WBB E7 ended up being about $100 less than the 3rd Rail version, and a LOT more work.
I'd still have to upgrade the WBB E7 to get it up to TMCC, so the end cost may even be more for a less detailed engine.