Joetom
Rolls Royce took roughly 75% of all 757 sales but the total no of engines was roughly half as the US airlines that took the PW's ordered them in big no's.
The E4 version was at one stage the most reliable jet engine with the highest on wing time.
The RB211 524G (i think) took only two orders on the 767 as it was a late player to the game and only BA and it was either Royal Brunei or a Chinese airline that ordered them.
Out of runway
Not quite true. if you look at it in number of operators then yes as only Four to the best of my recollection took them BA CX QF and Air New Zealand plus one or two others i cant remember.
On time
The E4 was chosen by many airlines for its reliability mostly in part to its three shaft design and if i remember correctly at one time it was the ONLY one of the two certified for ETOPS.
Rolls Royce powered 757's are in vogue at the moment as they have a proven ETOPS history and are very reliable. I believe their sfc is a little more than the PW's but have more power (please correct if wrong).