My preference is to split the difference on bag size and use a 75lb semi closed circuit bag. It is larger and more visible than a 50 but a pair of them pack much easier on the bottom of the back plate than do 100 lb bags. I also like the semi closed lift bag design as it will not dump air on the surface, yet can be filled in a variety of ways and the top mounted dump valve allows them to be used as backup floatation on ascent.
That said, I have SMB's, 50 lb, 75 lb, 100 lb and 200 lb lift bags to allow me to use what ever the specific dive will require. I'd rather specialize when possible and only rely on the "best compromise" bag/smb on dives where more versatility is anticipated to be required.