First of all, I think that the Inco 625 is the best choice of the two. If this is cracking during the weld process, the probable reason is solidification cracking due to the lack of ferrite to help eliminate cracking. Remember, the 35Cr-45Ni is a pure austenitic that is prone to hot cracking if not carefully welded. Another reason can be that the current tubes are experiencing some sigma phase (embrittlement) due to the constant exposure to the high heat.
Are thise tubes butt-welds? What weld process are you using? If using TIG, are you able to purge the welds?
If we can get some more information on where the cracks are occuring, and if they are transverse or longitudinal caracks, we may be better qualified to answer your concerns. Thanks..

Chuck