For what it is worth I am a fan of the Rapha Core line of bibs. I have Castelli and Giordana as well as Perl Izumi and the random $30 bibs off of Amazon (Prazeleski or some such) from when I first started cycling 6 months ago and was not sure how I would like it or if I would stick with it (thousands of dollars later and my N+1 is currently kicking in hard and my 3 bikes seem like not enough... lol). I like the Castelli bibs but the size differences and fit is wildly different between lines and I have heard even between years/generations. I use the cheap bibs for trainer days and the Rapha Core Bibs have been my go to for outside. I live in Sunny, Hot, and Humid Florida if climate matters to your decision. I have been a fan of the Rapha Core Cargo Bibs because they have 2 side mesh pockets that lie flat, but I really like the 2 pockets in the back above the pad (they are covered by the jersey and sit about where your mid-lumbar region is). These great for putting your phone, wallet, keys, etc and not working about them getting airborne from the jersey pockets. The Giordana bibs are FR-C and nice as well. But, at $150 for the Rapha bibs vs $250+ for most Castelli and like brands I have no problem with buying the Raphas.
I am ~5'10 178lbs and wear XL in most of the Castelli Bibs (L to XL in jerseys), M to L in Rapha Core Line, and M to L in Pearl Izumi, XL in Giordana