Help me pick the best Medusa Option for my CRT .5

So I have a CRT .5 and after busting the shaft on my MM5700 EVERYONE suggested a bigger motor. So here I am ready to pull the trigger. At the moment the only lipos I have are a maxamps 6000 3s and one 2s same cap.
I will buy one more lipo maybe a 4s zippy if needed but would be nice to be able to utilize at least the 3s in the meantime of me making some more casholla.

Just tell me what size to get so I dont mistakenly make the wrong expensive mistake.

Thanks guys...Ive been crawling quite a bit lately and have a go fast itch. Plus everyone in the crawling crew has slashes that flip over on every pass almost. It would be nice to whoop up on them with my brushless CRT .5 since they no nothing about them.