The thing is that SATA Express won't actually be a huge amount faster in normal workloads - you'll be able to write data faster in sequential, but there's nowhere to get 500+MB/s from anyway, except for video capture. IOPS won't be significantly improved.
I expect you'll find that buying a Z87 MB now is a cheaper option, as you'll need an H87 MB in the meantime, which is another $70+ likely.
Also, SATA Express SSDs will be overpriced I expect, and quite rare until Broadwell is thoroughly entrenched. Especially if the actual SSD isn't backwards-compatible.