Maxwell Screens.
My story: I bought a trade-in Rolleiflex 3.5 at my local store. It was delightful. Then, for some reason, I bought a fully-overhauled near-mint Rolleicord Vb from Harry Fleenor [ rolleirepairs.com ] and had him put in a Maxwell screen [without microprism, with grid]. When the 'cord arrived, I found the Maxwell screen significantly more pleasant to use, and found the Rolleiflex so dim that I had little desire to use it. Since then, my Rolleiflex film advance seized, I had it overhauled and a Maxwell screen installed in it, too. Now: better than new.
Summary: Some say the Maxwell screens are 2 stops brighter. I'd agree. The corners are especially brighter. Without using a Maxwell screen, obviously generations of Rollei TLR photographers did superb work and never complained. They are not cheap, so that must be considered. On the upside, Harry adjusts the focus of the taking and viewing lenses so that both are synchronized, He states that they often aren't correct when he receives Rollei TLRs.
Your next question: Overhaul turn-around time? For me: [1] Rolleicord and [1] Rolleiflex, sent together, full overhauls on each, screens in each, four months, inc. shipping both ways. I'm pleased.