These are the same light, but with different batteries. The 400R uses an external battery pack, so you have to find a place to mount both the light and the battery pack. The 300R is entirely self-contained. It's just thicker to hold the batteries.
I own a 300R. I put a mount on each of my four bikes, and move it from bike to bike. It uses standard plastic reflector mounts. While it comes an assortment of mounts in the package, all my bikes use the same size. I just get more at the LBS. They don't even charge me. This also means that the light cost me $50 per bike, which is a little easier to take.
Meanwhile, I've never had my 300R go into low power mode, let alone run out entirely. I charge it every Sunday afternoon and it lasts for the whole week. And that's with using it on every ride--day or night--from 50 miles a week in winter to 150 miles a week in summer.
Same experience here. Its a great light from a great company. The only problem I had with mine was the mount - the nut tends to strip the plastic out if you remove it too often (like I did). A call to Dinotte and they shipped me a new mount free of charge.