My experience with PUGs is that you get a decent one about 1 in 5, meaning they'll be competent and sufficiently geared to complete the mission at whatever difficulty. If you get into a shit one, you can usually tell straight away and drop out.
It is, however, a much better experience with friends. One reason is that PUGs do not tend to communicate much. It's okay because I honestly don't feel like chatting with random tards any more than they do so I just use the coms wheel 95% of the time. But, with friends, you really get into a Shoot, Move, Communicate zone that takes the teamwork fun to a whole new level. Like at difficulty levels 8/9, constant coms and cooperation makes it a lot more enjoyable.
For example, you may decide you're too low on reinforcements to try for extra objectives and you just need to bail. Your friends will probably be able to reach an agreement, pull the plug on it, and head to extraction. PUGs, who fucking knows.
It's just a much richer experience with friends. Plus, with friends, you tend to just laugh at FF whereas in PUGs it can start to piss you off.