@Rush
I think the 125 cu ft tank is the perfect balance between size and portability. The tanks are fairly large, but I can carry them around without issue. I fill my E tanks off of them and also have them set up for home use. The rough conversion is that a 125 cu ft tank has almost 3500 L of O2 (close to the volume of 6 E tanks), and that can usually last me a couple weeks. If I have a really bad hit (or get hit at night), an E tank is not quite large enough to kill it. Having the extra capacity in the house and ready to go is of great comfort when you're getting hit a lot.
Also, I had a good experience getting an O2 Rx while traveling to Orlando a few months ago. I had absolutely no trouble getting E tanks delivered to my hotel room, and only had to fax them the Rx. The company is called Orlando Medical Rentals. They appear to be setup for serving vacationers paying out-of-pocket, so it might not be cost effective for someone who lives in the area. But I thought I would pass along probably the only good experience I have had getting service from an oxygen supplier.