I can't believe I'm investing time and energy in this but the honour of the Legion is at stake!
1) Whilst it's true that only three members of the Legion visit Superboy in that first story, they say they are members of a club - this suggests that the other members are out there somewhere. That would be Chameleon Boy and Invisible Kid (arguably, these two were with the original three on that first trip - how would you know), Colossal Boy, Star Boy, Brainiac 5, Triplicate Girl (don't like yours much), Shrinking Violet (mind where you put your feet), Sun Boy, Bouncing Boy (bouncy bouncy, oh yeah yeah), Phantom Girl (sigh - why does she never return my calls), and Ultra Boy. That's a pretty impressive line-up - enough to trounce those wimps in the X-Men anyway.
2) Okay, I accept Prof X could be a problem - Superboy/Superman was always a bit weak in the face of mental powers or magic - but the Prof is a smart guy - I think it's most likely he would ditch the mutants and offer to lead the Legion.
3) Of course the X-Men needed to develop and redevelop because they were so poor initially - but with the Legion, all that was required was a little trimming - Ferro Lad had to go.
4) I agree the Imperial Guard was a direct rip-off of the Legion. I'm surprised DC didn't sue - but I guess imitation is the finest form of flattery. To me, it was at that point that Marvel conceded they had lost the case and the X-Men (should that now be X-People) were inferior.
Hegelian dialectic dictates that discussion will lead to a new consensus. I fully concur - I'm sure everyone can all agree that both groups are at least heaps better than those part-time Avengers :>