I think more skill points should be obtained from friendlies than for training. One friendly perhaps should equal two batches of training. I say this because in real life, managers and coaches always say that NOTHING can compare to match practice.

To answer the OP - I always find I get more skill points against better opposition but that's outweighed by it taking away more condition and the higher chance of injuries.

I've never really thought of playing friendlies as a means of obtaining booster packs. Prefer watching the promo videos and for me I think power training is much more beneficial to improve players than playing friendlies.