i would say the 50# on a baitcasting reel.  the larger diameter prevents the line from digging into itself on a hookset and during the fight.  which will cause casting problems on your next cast.

I've been told that you need to put mono on first because the braid will spin on your reel and not get tight to keep line in place.  True? or not

I've been told that you need to put mono on first because the braid will spin on your reel and not get tight to keep line in place. True? or not

Yes indeed you need a backing, also line ratings for any reel pertain to line dia. not for the lb test.

I simply wrap teflon tape around my spool a few times and then put the braid on.  Keeps it from spinning just fine.  Normally I leave backing on my reels so I'm only spooling 100 yds or so of new line.  But with braid I'll do the tape, then a full spool, which I'll transfer to another reel later, reversing so to speak.  Braid doesn't wear out until it loses color.  This spring I transferred the Power Pro from my Calais to my Stradic and all is well.