eh... you've mentioned you're picking between aero steelix or m.gross?? those are three very different types of pokemon man..... and what are the other 5 already in the party? ... eh i'll give some general opinion...
steel is a physix sponge... it's got no other use other than that, but it's a really good one, if people were over using curselax (or any other variant of snorlax) even blissey (unless it uses thunder wave...) even tho blissies are generali tossers (literali) you could stil own it before it does any serious harm ... (but obviousli there are more effective ways to get rid of blissies and snorlaxes other than using steelix..)
aero's a very nice sin, high atk, high speed, if only it wasn't friggin rock... but that aside, aero's speed stats are impressive, but it's hard to keep an aero in battle for long unless it's combined with other specific pokes...
metagross has friggin almost-uber stats, its the pokemon with the highest atk that can learn explosion... so if you ever think met's going down no matta wat u at least take em with you... other than that, it's only significant weakness is being a tad slow and weak to fire... both of which can be over come by moves learned naturali by it (c.mind, agility)
... n e wai i think thats all i'd need to know to choose between them... (given i already know who the other 5 are) if you've already got sins i doubt u'd need aero... physical sins are crap unless they're BLARDI powerful (nothing falls into this catergory) rather than using steelix people tend to use skarmory coz it's a flyer... and it also doubles as a spiker... so i guess... metagross would be yer choice... (althought i don't like it that much since i see it too often...)
Frank