General question about my Roth IRA

[arsoisaen]

im a little confused on how to allocate for my roth.

i understand how the tax deferral works, but lets say i have equity fund X which i believe will return 10% over the next 50 years and bond fund Y which will return 5%. X is a very tax efficient fund while Y has a lot of taxes to pay.

do i choose fund X to put in my roth because since i believe i will get a much greater return, and therefore the tax benefits will be better? or does this thinking not work and i should just put the least tax efficient (Y) fund in my roth at all times?