I don't have either, but out of the two I believe the Tec arc has a reasonable rep, whereas the SIP units I think are brought in from China with relatively poor backup from SIP.
I have an Rtech, which is often comparable to the Tec Arc. Tec Arc has better pulse frequency (on DC) range, but RTech has variable AC frequency.
I also have an R-Tech (200 amp ac/dc) that I picked up a little over a week ago as my first Tig. As a total newbie I am very pleased with it, it has a really nice smooth arc and all the functions that i'll ever need.
Very nearly went for the Tec-Arc myself but was swayed to R-tech because of the supposedly superior IGBT power module (As opposed to Mosfet's), and the fact that it came with a foot pedal included whereas you have to buy seperate with the Tec-arc.
bit the bullet yesterday and and getting the Tec-arc 206 with foot pedal for £1000 pic it up on tuesday.
so will be in my shed for a few hours most days now . ( that should please the wife not. lol)
just got and been playing with my new welder what a improvement to my old scratch tig.
got a good deal from my local shop £1100 inc vat, with foot pedal and gauntlets n sleeves plus a 1kg pack of steel and a 1kg pack of 4043 alloy rods.
just waiting for my foot pedal to arrive should be here on wednesday so i can have a play with some alloy welding.