I don't think the RRS clamp has a spirit level. I use Markins screw clamps on my Markins ballheads as well as my Linhof and Arca-Swiss heads. I think the Markins clamps are very well designed and built, and I particularly like their added safety feature; a spring-loaded safety detent pin. The reason I prefer screw clamps is that I use plates from a number of different manufacturers and the screw clamp design handles that situation much better than the lever designs.
Actually the RRS lever clamp doesn't adjust to slightly different plate dimensions but the Markins lever clamp does, although it is fiddly. Also the RRS lever clamp weighs 150g, almost twice the weight of the screw clamp on the Q3T which is 78g, and with a travel tripod weight matters. So is it worth spending the extra money? RRS stuff is well made and looks nice and their marketing is great. But I've used Markins clamps for 4+ years and they work very well indeed; never a problem and they work on any plate I care to buy. So I at least don't spend the extra money... 
There is no disadvantage to L-plates over universal plates, except for the obvious one that an L-plate is designed for a specific camera and will not fit a different camera.