Buy!
I have a MKOL 4.5kg bottle that I got when they first started, over 15 years ago, obviously it hasn't accumulated over $1500 in rent in that time, could still be sold for half of the new price, so I figure I'm a long way ahead.
I would happily recommend one of MKOL's bottles over anything else on the market, the build quality is the highest, the aluminium bottle makes for the most convenient weight.
Lots of sizes to choose from but I like the 4.5kg, short enough to go anywhere most kegs will go, wide enough to be stable even with a double reg hanging off the side, carry handle... right answer for me.
Mark