the loaders are all gpt, if it would be a loader gpt vs. mbr problem is would not boot at all (mbr loader is often needed with older hp desktops and only if uefi boot is not working like with loader 1.03b), if you find you system in network its not a boot problem
3617 uses the same kernel as 3615, just some minor kernel settings different then 3615, never seen it behave different when installing, most likely usb vid/pid wrong, sometimes it can be a problem if the disk is not empty (have partitions on it)
if 3615 works for you dont bother, 3617 only supports 16 instead of 8 cpu cores and has some newer lsi sas drivers