powcon 300 sm vs 300 st

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Hello, my name is Yum Chinn and have recently joined this forum,I recently saw both  the 300 st and the 300 sm on Ebay and bought the 300 sm because I already have a constant current source for tig, and I have been told that the 300 st is no good for mig, whereas the opposite is not true.  Is the 300sm more or less constant voltage and/or constant current and the 300 st only constant current?  Also, I remember an old post where someone was going to use the pulser RC250P with a Powcon 400 sm.  I wonder how these units are interconnected.  According to Arc Products, the RC250P can only be interfaced to the 400 smt.  I know people have used the 300 sm for tig, but since there is no capacitor start on the sm, does one have to scratch start? Or is it possible to install HF on top of the Powcon?  How often does one need to use the pulser for either tig or mig?I have not received my Powcon yet, and have yet to shop for my wire feeder and mig gun, otherwise I may not have to ask so many questions.  Thank you.
Reply:Yes, you can put an HF box on any CC stick machine that I know of to get easier TIG welding.I also have the PowCon 300SM, and love it.  I believe you are correct in all your assumptions about CC v. CV on these machines.Are you set up for 3-phase, or are you using them at single phase?
Reply:Thanks MAC702 for the info.  I do have 3 phase as well as single phase in my shop.  I was concerned that the HF unit may interfere with the internal electronics of the Powcon, mine is an old spark gap variety HF.  Is the powcon good at low currents for Tig?  Say for 1/8" or thinner mild steel?
Reply:Ian at Arc Products will be happy to answer all your questions.  I have a 200SM and he has been generous with information.  I know that ST signifies Stick/Tig and SM stands for Stick/Mig, but I have heard of people using the SM for all three.  I have recently seen several PowCon Pulser attachments on Ebay also.