I have had a lot of experience with this, this past week... But it's still just my opinion, you have to do what works best for you!
Rondo is super easy to use, but it goes on a bit thick and has pretty miserable strength. If inner size is an issue, go with fiberglass. Two thin layers will give you good strength.
I cracked my rondo-only helmet earlier this week, from putting my arm and not much pressure on it. My all fiberglass helmet I can sit on, though it has three layers of glass and a layer of rondo.
The downside to glass is it is REALLY a pain to get it into cracks and details of the piece. People go rondo if they're not going to be too rough with their armor, or if strength isn't an issue, and they want to get something done quick and easy, as rondo alone will probably take an hour less time in work, and two hours less in curing, than fiberglass because it's liquid, and the two catalysts make it set at bondo speeds which are faster than resin speeds.
A really fantastic compromise for strength and ease is to first glass all of the easy to fiberglass areas. Then use a batch of rondo to fill in the details. If you use enough rondo, you can smooth out and cover the details enough to easily put a layer or two of fiberglass over the rondo to reinforce it. Honestly though, so long as you are careful with the rondo, and lay the fiberglass down flat, you won't be getting a whole lot of space taken up by the stuff, unless you sized the helmet to just fit your head.
Bottom line, fiberglass will always win in strength to thickness, but rondo is easy to use and can get into crevasses like it's nothing.