WEld in subframe connectors are a must for any mustang. Steeda's subframe are made from round tube steel which is stronger/stiffer than the rectangular tubing, but if your car is lowered to much you will have problems with scraping. I personally have Maximum Motorsports full length sfc's. I love them!
i would say subframes are a little extreme for most mustangs, but they are a good addition if you have a STB, f/r sway bars, phb, tq arm, etc. it's just a bad idea to have them bolted on because when it catches on something it will rip it right off. mine were nearly ripped off and mine were welded. i got some off of ebay for about 40 bucks and welded for 90.
the tubular are stonger, but the rectangular provide for more welding surface, thus making it strong.
i have to disagree with V6powerstang on the importance of SFCs. without the unibody structure of our cars, there is a lot of flex in the cars upon hard launches and even hard turning. when nice and modded, one can see .1 off your 60's just by tieing the two subframes together.
they help with suspension somewhat, but IMO are better for all around driving. some suspension, some chassis support, some off-the-line go.
If you are going to install Steeda subframe connectors and they didn't come with cross braces, order yourself a set and have them put on when you get the connectors welded in, you can't add them later, I found that out the hard way, lol.