My Disco is and will probably be my only

PCP, fingers crossed LOL.
My last PCP I only had a pump and I didnt want the added expense. hated the pump, so I sold the gun and pump.
when I bought my Disco, I liked the idea about using a pump with very little effort to charge it. Reading all about the Disco pumps going bad all the time, I invested in a 30cf 3000 psi rated scuba tank and along with a fill gauge bought off fleabay, I only have an extra 150 invested, just a little less than another, better pump.

Doesnt answer your question, but, your first consideration on 3000 or 4500 tank, ya have to check if there is any one that can fill to 4500 first before you buy. My 3000 fits me just fine, for now, because like you, I only plan to have just this one PCP, I too am a 02 and springer guy.
Being portable is another question. once you get a 88cf scuba tank, which is not very portable from what I've seen, CF tanks are, but at a even higher cost.
So its up to you how far you want to go.

Buster
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one step closer to the ultimate goal