Wow, umm..
I guess I prefer MLL. Both are equally crowded, UC arguably more, but on my entrances it's pretty similar. I don't really care about the bartender thing, at the very least that seems to keep things a bit cleaner.
I like the standardized desks at the UCs and the wifi is better. 50% of the time I have to reconnect/reconfigure to get connected in the MLLs, sometimes it never happens. Sometimes it kicks in right as I open my laptop. It's too hit or miss. The UC wifi has 4-5 options to log in...none of them work except the daily code for me.
The food in both bothers me, I really do wish they had some cold cuts and bread and I could make a simple sandwich...but the MLL does have better options at the right times of day.
I'm fine with the free booze options at the UC, I'm more bummed that they usually use tiny glasses.
In the end I think it's basically 50/50. I like having access. I wouldn't pay as a UA member to have access, I doubt I would pay to have access to the MLL either. Maybe via credit card if I didn't have status (which I won't in 2017). I could toss in Avianca and Copa lounges, none of them are great. I like the tiny sandwiches at the AV ones, but I hate that you can't get booze until 1600. I like the Copa Clubs, PTY gets super crowded though and virtually no food, MDE had almost no one in it when I was there, little snacks, and a bartender, but I had drinks early in the day so that was nice.
In the end, none are great (that I've been to) but I'd certainly rather have access than not. I suppose I prefer AA clubs, there's some free booze, some snacks but the best part is the pay food. If you're not gonna let me bring food in, that's lame but whatever, but then at least let me buy a meal.
Oh and as someone else brought up, I love that the UA agents at the UC can handle changes. Sometimes they're even just nicer because maybe they value the customers more than the service desk does.