I own a full commission RS TK (PVC) and a full RS TB (ABS) with the Red Label helmet.
My own two cents.
The TK PVC is the way to go, and I had the same concerns over yellowing. The important point is to always keep your costume out of UV exposure and climate controlled. You need to be careful with PVC, in it as it's not as felxible . I've had my suit for 3 years and no cracks. It looks great, nice and shiny and won't yellow. Super durable if you treat it right. This is my go-to and my wife's preference, but you will need a handler to get suited up.
The TB is very "off-white" in color. It's ABS, but the plastic is not painted and very much a semi-gloss and so off-white it's almost heading towards egg-shell in color tone. I got it clean and instantly regretted it, as it's so off white that it really doesn't look right compared to shiny true white TKs or TBs. Depending on how "true white" the soft-goods are, you may almost look two tone. Thus, weathering to make it blend is pretty much a must in my book. Outside of some basic help to get my flack vest on, I don't really need significant help from a handler to get in and out of costume.
The red label TB helmet is perfectly serviceable and in my opinion prefered as the construction will be far more durable than the "true to screen" construction. And yes, it was me who had the helmet and it came in a true white and my armor was egg-shell. They paid for shipping and they sent me a new helmet right away that was color matched. When you order, add a note that mentions you would like the helmet match the armor, you can then follow up with an email to RS and they will confirm they received the note on the order. I've never had an issue with these guys in terms of service or communication.
I like the RS kit enough that I purchased a second set of the hard parts for my wife. I went with another vendor for a full commission of the soft goods to get a custom fit.
Please feel free to ask any additional questions. I'm also happy to send pictures if you'd like. you can see some pics on instagram here:
https://www.instagram.com/james_that_501st_guy/