EF Adapter ... P6 vs. M42 ?? opinions please

Tonight, I bought a new lens .. Zeiss MC SONNAR 2.8 180mm .. will arrive next week.

It appears to be a Pentacon 6 / Kiev 66 mount with an original Zeiss M42 adapter. Here's my question to a few of you who may have tinkered with adapted glass before. For a Canon DSLR, would you keep the original adapter on the lens and buy an M42 > EF adapter for it, or remove that adapter and get a Pentacon 6 > EF adapter instead?

Thanks.

I'm not sure if there's a benefit either way but I use mine with a P6 to EOS.