Mettaton NEO has low DEF, but high ATK. Sans has low both, but KR bulldozes through HP like 1000 degree knife through butter. Sans can dodge. Mettaton can't. But mettaton has a gun. So... idk
Yeah. If it was box Mettaton, Mettaton would win, just by wearing Sans down with his unbreakable body. If EX, Sans. Mettaton would put up a good fight though. also, sans would hate the disco ball attack.
I'm pretty sure that, in most chances, regardless of what form Mettaton is taking, Sans would eventually get really tired. Although I believe that Sans could catch Mettaton when least expected. Who knows.
I'll say Mettaton NEO. Mettaton NEO has low DEF while Sans has low DEF AND ATK. Sans also has KR but it won't really work on Mettaton NEO. And Mettaton NEO most likely got beaten by Frisk just because they already had a high LV and high ATK.