I usually find my small-rackers work well as team players, but not as stand-alone units. The double-bigboy is going to have more full damage packages on target, rolls faster, and I find they seem to work better as a main gun.
Basically, it's not so much whether I want to get one big whack or not, it's that the big racks play faster and still do appreciable damage as a standalone system. Multiple small racks slow the game down - eating up playtime for little appreciable benefit (which is okay in Megamek, but unacceptable in the tabletop) and put out a lot of stray 1-3 point sandblasting shots. They make for a good auxillary weapon that can occasionally put some hurt on a target. They just don't handle extended combat as the primary weapon as well as large racks. They get more shots per ton, yes, but that can be a serious liability if you only take one (3050 SSRM designs, I'm looking at you)
For LRM racks, my two favorite rides (the Bandersnatch and Mad Dog) are at opposing ends of the spectrum: the Bandie has 3 L-5 racks, and performs well with them - as supplemental weapons. The Mad Dog has (usually) 2-4 really big racks, and can tear things all to hell and gone with the missiles alone. I tried running an experimental Bandersnatch with 40 tubes (stripped the Mlasers and a ton of LBX ammo for more LRM-5s) up against both Archers and a Bandy variant with LRM-20s as the main guns, and the latter always seemed to actually open up the armor and kill faster. Sure, there were a few lucky crits, but I don't base my tactics on Hail Mary plays, and over time the matches were still averaging out to be about 45% (small rack)/ 55%(huge tracts of land). With other units in the lances, it slid right back to 50/50.
Multiple small racks is still a good choice for a Medium or Light design: it keeps your eggs out of one basket, and they're intended to be team players more than the Heavies are.
My rule of thumb is, if there's more than one rack in a single location, just round it up to the next biggest rack unless there's a darned good reason. If you have 3-4 racks on a single design, but they're spread out (Jenner IIc, Bushwhacker, Commando, Whitworth..) there's no reason to combine them into one bigger rack but it won't necessarily hurt the design's effectiveness (Griffin, Centurion, Uziel-B)