Size of workbench is important (it's always about size isn't it?) for a stationary/portable/collapsible design. So is the expected type of work being done. Welding and a wood top don't mix very well. Neither does wood and solvents/oils. Wood top for woodworking or fragile components is ideal. One thing I've tried to do is have different benches for different types of work but it takes up a lot of precious floor space. For general assembly work that's messy I try to use discarded kitchen/bath counter tops. They're fairly flat, clean up spills real easy, non-staining, sturdy, and often free if someone is remodeling. The shops/stores/contractors that do remodeling often haul the old ones away to throw them out, they might be happy if you offer to take some away. Any plywood, MDF, or wood top is going to need sanding/sealing or it will generate splinters/fibers as it absorbs humidity/liquids. A Formica countertop is already smooth, no finishing required. Some may not like the appearance but it depends upon what you use. You could try to incorporate wood, Formica, and a SS panel into the top and have all 3 surfaces for welding, assembly, and woodworking or just have removeable sheet surfaces that can be stored under the bench.