So many good points. To the op, it all comes down to what YOUR use needs are. Think in terms of what your storing, and how close you want and need to be to these tools. I try to keep the things I use the most in the closest most centralized easy to access places. Buying another top might save a few steps grabbing tools, buying a bottom provides counter space, but takes up floor space. Cost difference per storage inch is insignificant, how you make use of space is. Making good use of your space will save you time, which is the same as saving you money.
I have the 44 bottom with MDF surface and I use the top of it a good bit, since my workbench is on the other side of 2 car garage and my tool cart only catches so much. Lucky for me I have plenty of tool storage space now and don't have to consider a top box. I wasted a lot of time waiting for my CM 26 combo boxes to go on sale and I had to return my first HF 44 due to a dent. If I were to do it all over, I'd go with Extreme 55 or 56 combo, HF biggest tool cart, or 2!, a nice 8 foot workbench and call it a day.