Plywood versus Valchromat (or similar): I'm not convinced it's any better. The ply table I have is very well made, but it is a little "fluffy" and needs a finishing coat or something similar. I've put machine wax on mine as I didn't want finish to alter the holes. That seems to work well. The aluminium dogs sold by CNC Design are a very snug fit anyway, so I trust its squareness. I suspect that Valchromat will last longer and move around a bit less, but both are pretty stable. You could do worse than ask CNC Designs what they think (they use their own tables!).
Storage underneath: This is a toughie, but don't go wild. A huge part of the usefulness of an MFT is the ability to use clamps through the holes rather than just along the edges, and you need room for those clamps to stick out underneath, and/or room to get your hand in under there to position stuff.. I don't have many holes in my bench*, but I store my welder underneath it, and too often I have to pull it out to allow a holdfast shaft to drop through enough to clamp the way I want it to.
I was all set to buy the Axminster MFT frame a while back, but three things put me off: (1) when assembled it's rather heavy and awkward (the table I have breaks down to simple, flat pieces of plywood), (2) it relies on trestles that need a flat floor - my workshop floor is rough concrete and this wouldn't work, and (3) it's a box construction, with storage inside.
At a first glance that looks good, but then you think about it: once you put a workpiece on top, you can no longer get at the storage! And then you can't use long clamps on the top either, as there's no room. So it's not anything like as useful as you might think at first.
On balance, I would have some storage underneath, if possible, but it would be a rolling-cabinet arrangement , that could be pulled out of the way quickly, and I'd leave at least 8-9" of gap to the underside of the MFT top, so you don't need even to do that most of the time.
The table I have is well thought out. There are horizontal slots, so you can slide a rail into a handy place while you secure the workpiece or whatever, and a couple of narrow shelves with dog holes, to store dogs, clamps, etc., in easy reach. It looks like it was designed by people who use such things! I am really, really pleased with it, although I have to dispose of a motorbike before it gets a semi-permanent spot in the middle of the garage/workshop!
HTH, E.
* I presently use the old Record Holdfast system, with collars sunk into the bench, but it's not flexible enough and I'm going over to the more traditional holdfast approach when I eventually rebuild it.