Certainly it's all personal preference and I've tried different combinations but for me I ended up preferring just one large roll cab and that's it. I've worked in typical full service automotive repair shops over the years and I ended up finding have the large work surface to invaluable. I've had metal tops and butcherblock tops, I think I slightly prefer the wood top but it's basically a wash. Metal will look nicer longer but I love the look and feel of working on wood. I would say wood for not hard use and metal for more heavy duty industrial work.
The downside of this is that pretty much every time I came back from days off I found that others have used my box for storage. Mechanics can be slobs and will set something on any flat surface that's nearby. After one things gets put there, forget about it, **** will pile up. A material cover didn't help either. Having a top box would eliminate this one, petty nuisance.
Oh yeah, having a vise on your box is nice too but this also leads to others using it and possibly damaging your rollaway. This happened to me before so I quit using a vise on my box at work.
Good luck with whatever you decide, for me, just a bottom box works best.