The digital noise is probably because the two cards do not run on exactly the same frequency. They will hence "slip" compared to each other now and then. The simple solution is to make them run from the same clock -- simple set one as master and the other as slave and transfer the word-clock between them. That is what the WC in and out connectors are for.
mtc is strictly for synching where you are on the "tape" so to say, the big picture. It does not carry word clock
spdif can be used to carry a clock signal between two devices, it contains word clock. a proper word clock on BNC-s is often more convenient though
synching will not help with cpu load, sorry.
Gunnar
PS: if all you want to do is to synch two units, all you need is one (1) cable. Word clock was designed ages ago to have something called termination, in effect a resistor placed on the last of several units daisy-chained on the same cable. For a very short cable run this is generally not needed and many sound cards does include termination on the IN connector anyway, so wait a bit with that if you are on a tight budget. If not, get the T-connector and termination resistor at the same time you get the cable.