Well, the best way to preserve running fitness is to run (Specificity). But as that can get stale, the main question is, for triathlon, what is going to be a better cross training tool from running? cycling or elliptical training. You cycle in triathlon, so it would be more logical to stay trained in that area.

It has also been argued that cycling helps with your turnover speed for running..... I can see with the fixed movement in cycling where his argument is coming from, in addition it can tighten hip flexors and possibly hamstrings---as also running does, though. Just make sure you add flexibility into your program and you will be fine. If you primarily run to begin with and use cycling as more of a cross training tool, you should have nothing to worry about.