I say Gym Style, if strength if your goal.
There are more exercises for each bodypart in the GS series, so you can hit each muscle group much more thoroughly.
In the UB Pyramids, there is only 2 exercise for each bodypart, except shoulders which has 3. You are lifting different amounts of weight for those exercises, but it is still only the 2-3 variations/bodypart. Also, it does move a little fast, without many breaks in between sets, so the workout feels a bit more endurance based than strength to me.
It is definitely a challenging workout, but for strength gains and more of a fun factor GS is the way to go, IMO.
Take care, Lynn M.