Eric - I do indeed have some thoughts on this, and a BIT of experience. I have done a table much like you have described in one of your options - described in this post earlier on NCWW:
Padauk table - bar height (Post 1 of 3)
This is a 4/4 wood top and I doubled the thickness on the edges - not for strength, but for appearance. 4/4 wood, even once cleaned up, is plenty strong for a table top - assuming normal use. But given the industrial look it seems like you might be aiming for with pipe legs, my judgement would be make the top (appear) thicker than 3/4.
- I have not seen hairpin legs on anything except (low) coffee or end tables, and I doubt that I would choose these for such a tall and large table. I expect they would NOT be stable enough for a dining type table, unless they were very stout (and therefore not hairpin-ish).
- Hairpin legs typically do NOT have any interconnection of the legs , which will make the table quite wobbly I expect (pipe legs on flanges MIGHT not suffer from the same issue).
- I don't know how you are planning to use these tables, but I expect you'll find that without footrests (cross bars) that sitting at this table might not be as comfortable as you might expect. Mocking up the base with scrap wood, 2x4s, pocket screws, 1/8 luan plywood top etc is something I ALWAYS do for new projects; makes for a much better finish project (when I do my own design - which is pretty much always). In this case I suggest having the stools or chairs available (if possible) to get a feel for the dimensions and whether you want or need a foot rest (I expect you will).
- Consider is that a 42" wide wood table needs a way to accommodate wood movement - width changes with humidity changes (expansion/contraction is across the grain) . So whether you use plywood or cross bars or any or structure underneath, or not, ensure that the structure allows for width changes. There are ways to calculate the max movement for each species, but the common ways to accommodate movement have always worked for me in normal sized pieces, without determining exactly how much movement I need to allow for.
If you can use sketchup or other drawing programs that can provide a better view of the look of the piece - scaled to size. Larger tables need larger sized support structure - to my eye at least (i.e. no hairpin legs for me on a large table). A mockup then provides instant feedback on usage and comfort, as well as aesthetics (although I seldom make a complete mockup with a full thickness table top - I just grab some plywood/cardboard for that and then use my imagination - thankfully my wife can see past that and imagine too!).
Hope that helps. Keep us up to date on design choices and project build. Is this going into commercial space or do you just have a large party room at home?