The problem I had with Sketchup was that if you wanted to go back and change a dimension in your design, it would require a lot of rework. Also the free version does not include booleans. In a lot of cases you could figure out how to work around this, but it was a huge hassle. The last and biggest gripe with Sketchup is that it can be hard to get 'water tight' solids. It seems basic design operations would cause havoc and I spent a lot of time using plugins to debug small triangular holes and inverted surfaces.
OnShape let you define user settable dimensions on things that can easily be changed later. The design is essentially 'replayed' using the new value and all subsequent design steps adapt to the new values. Also It always creates good solids and the free version is fully functional. You just are limited in how many private designs you can have.