The GX355 is only 18 HP vs. 22 in the 455, (That's about 20% less power). Although they both have Yanmar diesels, it's an aluminum V twin in the GX355 with cast iron cylinder liners vs. the cast iron block inline triple in the 455- the triple being the more durable engine.
Tractor-wise, the two models share very similar features though.
Frame size and weight, engine durability, all shaft/hydro for the 455 and differential lock all favor the 455. Although the 355 is nice, the 455 is a much more durable and heavy duty machine.