Excuse my ignorance, but is a dumbbell not just a dumbbell? I can understand paying more for good bench but why would you "outgrow" weights? Yes you would lift heavier but that has nothing to do with not buying from said competitor?
basically standard weights are useless if your serious, if your a part timer looking to waste his time and money, then go ahead.
after 2 months of becoming serious about lifting (not missing a single workout) i was squatting 180kg, deadlifting 140kg, etc. this isnt normal, but i have always been naturally strong especially in my legs, but as you can see, i would have outgrown standard plates in a matter of weeks.
if all you want to do is arm curls then fine, go ahead get the catalogue stuff, but you would be better off just doing press ups and other bodyweight exercises.
ive spent over 4K easy on my home gym, and half the stuff was second hand. i also dont have a single cardio machine, its all weightlifting and a small amount of boxing gear. if i thought i could get away by spending a few quid down my local catalogue shop, then thats what i would have done, but it didnt.
today i lifted well over 1500kg today, not all in one go, but moving my weights back and forwards (i was spray painting the stuff i got second hand, they were in a state and i have been, refurbing them).
i have (using my memory here, never really wrote down)
6 x 25 kg plates
6 x 20 kg plates
4 x 15 kg plates
2 or 4 x 10 kg plates
4 x 5 kg plates
4 x 2.5kg plates
4 x 1.25 kg plates
all olympic, only 8 of the above plates were bought second hand.
2.5kg dumbells all the way up to 45kg dumbells in 2.5kg increments. all of this was bought second hand but the 12.5kg to 25kg dumbells are rubber hex and in brand new condition. i only had to paint the rest.
if your serious about lifting and or bodybuilding, you have to buy olympic gear.
if you want dumbells, you have to buy ironmasters, they are the only ones i can seriously recommend, bowflex are cheap and nasty (plastic components), catalogue equipment is just a waste of money.
im gonna try and find my thread i was making right now, these questions pop up far too often.