There are some nice improvements on the 251. I like the control panel setup better than my mts250s. Although I have no real problem with my controls. One thing I really like that I wish my mts had is more voltage ,30 volts is good. The mts should of had that much voltage available. On heavy plate I run my MM200 at 30 volts a lot. The MTS I have to take more time because I do not have the voltage with only 26.5v available. The HF is a good improvement also but the lift arc is not a deal killer. I would like the pulse feature for my spool gun on thin aluminum. It also is nice for vertical up short circuit welds. I can live without it but it does make for nice looking vertical up mig welds.