In most cases, the 68040 at 25MHz would be a little faster than the 68030@50MHz. The 68040 has much larger caches that greatly help performance with the 3640 but adds some incompatibility with older software, especially with games. The 3640 has no memory and accessing the motherboard memory is slow so the caches are very important to the 3640's performance. The "X-Calibur" is rare and would be too expensive to consider. The 68040 built in fpu is much faster at floating point than 68881/68882. If you have fast accelerator memory with the 68030@50MHz then I would stay with it for now. If you like the classic then wait for the Natami for an upgrade.
