Steel is ungodly expensive right now, but based the options you're considering you're wanting to do it right, so I'll just mention some thoughts, regardless of if it's home built, pre purchased etc, you want your welding table to essentially be flat as possible, even if it's got a minor enough bow you can't notice it with the naked eye, it'll make getting truly precise and square fit-up a nightmare, especially for longer parts. The second part to that though is excessive heat makes flat metal not so flat anymore, and it's very much possible to heat up a thinner table to the point it'll take a considerable bow by running numerous practice beads etc. I'd be very concerned about dropping the money on a 1/4 certiflat or other fab table, then padding beads with some 7018 and especially for example practicing aluminum welding by padding beads on a piece of aluminum which is going to much more readily deposit the heat into a more concentrated spot on the table and be even more likely to cause warping. One trick I'll use which admittedly was only such a no brainer because I got it for free is stick an aluminum top over the table when practicing or when you know you're going to be doing an unusually extensive amount of welding, it'll not just help absorb some of the heat but it'll take the rest and distribute it much more equally over the entire table, thicker is better obviously and I have a couple 2x2 foot sheets of 3/8 aluminum I was allowed to take home from work, but even as little as an 1/8th inch of aluminum will help with spreading out the heat massively. if you're stick welding and or otherwise don't want a badly gouged up spattered piece of aluminum, you can just overlay it with another piece of steel, or just use a sacrificial steel topping instead to begin with.
You might ask why bother and not just build a table that way to begin with or out of thicker material, well if I'm running beads and keeping in practice, I don't need a large table to do so, I can pretty much do so on a 12"x12" square especially with the original table underneath to prop on and most importantly it doesn't need to be perfectly flat/true to begin with nor stay that way. That way even if you can't afford an ideal solution of a perfectly flat table so thick warping won't be a concern, you can go with a lighter weight table for fab work where flatness and or fixturing points might be critical, then just throw a smaller sacrificial piece up on it for when you're going to be practicing or doing heavy welding rather than fab work.
Just in case it needs to be said though, a small square of 1/8 steel isn't going to do a fantastic job at distributing heat through the whole table or helping absorb it, it won't take long until it's so hot it's practically like laying the work piece directly on the table anyways, hence why at least a thin layer of aluminum is suggested in between or a somewhat thicker sacrificial piece. To give you an idea just how fantastic aluminum is at spreading heat out, as an experiment I took a piece of plywood and laid about a 14"x14" piece of 10 gauge aluminum, basically slightly thicker than 1/8th, then stuck a 1/4 steel coupon on top and stacked tig beads consecutively until the top part of the plate was glowing orange hot. I removed the piece of aluminum and there was not a single scorch mark or dark spot on it or even the slightest odor of burnt wood. not even directly underneath where the 1/4" coupon would've been sitting. I then ran a single bead of a few inches on a length 1/4 flat stock directly sitting on the plywood itself and immediately it started pouring smoke and charring underneath before I'd even finished.
Just to be more concise, the issue of warping rarely if ever an is a problem caused by the table as a whole getting too hot, for one if your table is getting uniformly too hot It'll get your attention real fast long before it approaches temperatures it might start to deform, want to stay propped on a 200-250 degree piece of metal? you'll notice it pretty quick, its that heavy welding is causing the work piece to heat up to the point that a localized part of the table is getting hotter far faster than it can spread that heat through the rest of the table.
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