People seem to prefer space bar profile but because that is not very available, custom split keyboards have most often used 2.75 and 2.25 ANSI-layout Shift keys. The downside is that the front edge is often perceived as begin too sharp.
If you use 2.25 spacing then you could use ANSI Enter keys as Space bars, which are flatter.
When we have had surveys on how people press the Space Bar, the results have been widely divergent. Some always use the left thumb, others always the right thumb (and Chyros is known to use his right index finger...).
BTW. On many ergo keyboards with fixed tenting, the space bar (clusters) is level where as the alphanumeric keys are tented. This is the case on the Kinesis and the MS "Natural" line, anyway.
