I would say get a KR808D-1 or a GreenCig cartomizer but then it won't be your spare, it will be your go to weapon. The m401 is a good hit with a long lasting battery as well. I have not had the m201 but if it's the same basic engineering and atomizer then it should be a fine choice as well.
I am done jacking around with carts and fillers all the time with the 3 piece models. I wish I had followed my gut when I made my initial purchase and gotten a cartomizer earlier than I did. On the flip side, I have tried a few different models and can speak from experience and not just parrot what others have said.
I agree on all counts. I have been there done that (with the 510, 801, 901, ladypipe) and all of them are now collecting dust. I get everything I need from my two piece models. I have both generic KR808s and Vapor Kings in all sorts of colors, auto and manual, short and long, and of course my beloved passthrough. Regardless of vendor, one thing remains true - they all blow my other devices out of the water.
I think I am about to get myself a GC to give it a whirl due to Snarky's beautiful review but regardless of my experience with the GC, I can tell you for sure, that my other devices will not be used again unless I find myself in crisis and they are the only devices I have left still working. Somehow, I don't see that happening.
So for now, it's cartomizers for me above all the other models that I have tried. This is it for me...unless of course some other new gadget comes out that I have to have. Hey I am a woman, I reserve the right to change my mind - and to look at you like you have two heads if you call me on it.