If the mortise is deep then an upspiral might reduce heat and clogging, but will leave a nasty top edge, not a problem for a mortise and tenon with a shoulder, but no good for an exposed top groove. In MDF, with a shallow groove it probably doesn't matter too much either way; straight or down spiral, but not an up cut bit. Generally I am changing to spiral bits because of the lower tendancy to pull out the grain while template cutting curves especially in end grain - bottom line, if you're going to buy a new bit for this job then make it a down cut.