I'm using the Blood Angels codex. As such I don't see why I can't publish points costs.
3x 5 man Combat Squads gives you 15 guys, 3 meltaguns and 3 Ld9 Power Weapons for 420 Points.
2x 10 man squad gives you 20 guys, 2 meltaguns and 2 ld9 Power Weapons for 430 points.
The question is, which is better. The first has 6 'special' guys and 9 bolter guys. The second has 4 'special' guys and 16 bolter guys.
Thing is as a bonus when you take casualties you are less likely to lose a 'special' guy when you have all those bullet catchers around.
So I would go with the full squads I think, especially if they were running at say Tau. Stinkin' Tau! On top of that if you taking a full squad you may as well throw in a Rhino or two.
Wait ... you need a full squad to take Special Weapons anyway now, don't you. Makes that an easy decision.