thinking about getting the 255 have had the 250 ex for about 4 yrs . and love the machine made some good money with it and it went down this week and need to send in for repairs and thinking about getting another machine and using my old machine for backup .so what are the benefits of having the 255 over the 250?why have all the different wave forms ,which is better for welding on aluminum,thats what i mainly weld.