Spline vs. Tabletop isn't really a direct comparison. It's true that a "tabletop" 4x8, or even a few 4x8s is a popular choice for a small home layout. I agree with your HO friend that using this approach to cover 400 sq ft isn't the best idea.
The alternative is some type of "open grid" construction, which allows the railroad right-of-way AND scenery to be built to varying elevations. An open-grid style layout will definitely have less plywood surface. The question becomes, how to support the track spanning the open joists? First, a point of vocabulary: When I hear "roadbed," I think of the cork or homasote under the track ties. What we're really discussing in this post is what goes under the cork; the substrate I like to call the "sub-roadbed."
Spline sub-roadbed is made by gluing thin, narrow strips of masonite (hardboard), decking, etc., together in several layers and setting them on edge. As I understand it, a chief advantage of laminated spline is that, compared to other types of sub-roadbed, it can be bent left and right into natural flowing curves, which nicely complement flex-track.
The main alternative to spline, which has been around longer and is probably more widely used, is long strips of 5/8" or 3/4" plywood traced onto and cut out from a large plywood sheet cut using a jigsaw or saber saw. The trick is to plan your cuts to minimize waste, while trying for the fewest cuts and avoiding unduly short pieces. You'll still use much less plywood compared to the table-top approach, because except at yards, industries, or towns, these strips of plywood will only be wide enough to support the track and roadbed, plus a little extra to hold the ballast. (If you're supporting a double mainline, the sub-roadbed has to be wide enough for both mains side-by-side, plus a little extra.) From videos that I've watched, tracing and cutting strips of sub-roadbed out of plywood sheet seems less labor-intensive than making splines. Also, compared to spline, plywood strips bend more naturally in the vertical dimension, so on smaller layouts it's easier to achieve gentle transitions from sloped to level track at the top and bottom of a grade.
With either of these methods, you'll eventually have to splice one section of sub-roadbed to the next, end to end. In either case you must take care to achieve a smooth joint, especially in the vertical plane, or you end up with a bump or hop in the track that could adversely affect operation.
Before purchasing any wood or tools, I suggest that you watch some videos about open grid benchwork and sub-roadbed. Alternately you could purchase a reference book on model RR benchwork from a major online retailer. It sounds like you have an awesome space with a lot of potential, and possible help from the experienced HO members of your club. Good luck with whatever method you choose!
