I do not know MySuit but I can clarify the things for Suitsupply.
What they call Blue Line Personal Tailoring is in fact a Made-To-Order suit in a given cut and a given size with a chosen fabric (of fairly standard S110's from Vitale Barberis Canonico) where there will be some tailoring in the factory after production (!) based on a few measurements taken from you (fewer than 10, I think) at the time of ordering. It's half-canvassed (as their RTW program), it's cheap but it's in no way a real MTM service. I would not recommend it.
Their Suitsupply Red Line Personal Tailoring is their MTM program, based on a size and a cut that the customer can choose from several available. Many details are changeable (posture (-> shoulder line), sleeve pitch, button stance and many more) as documented changes to the base size. These measured and discussed pattern changes will result in a customized pattern after which the suit is produced. This suit is fully canvassed, has real horn buttons and all the decorations and detailing of the RTW suits and you can choose your fabrics out of the excellent fabric books from Ariston Napoli. I have three suits made like this for me and I'm very pleased with them.
You won't get that for 500$, though, I think it starts at 700$ in the US and if you choose one of the more extreme fabrics (S160 etc.) you can pay more than 1000$. But for a fully canvassed MTM suit with that degree of detailing and a world class fabric from a very fine neapolitan mill I think it's a bargain anyway.
Thanks for the insight. So in other words when you buy a suit from SuitSupply, say this, http://www.suitsupply.com/grey-suit-washington/P2505W,en_US,pd.html?start=3&cgid=Suits its pretty much like going to Macys and buying a suit off the rack? Except that these suits are easier to tailor? So if I were to buy a Suit, I would still have to get it taiolored to a certain point, is that extra with Suitsupply? Also what is the slimmest fit? I am looking for something more fashion forward.