My wife and I each have a Roth IRA that we contribute the $4000 every Jan for new year. The IRAs are in two index funds with T Rowe Price. My question is: Which method of contributing would be more beneficial:

A: Contribute equal amounts monthly to the IRAs (aka Dollar cost averaging)

B: Continue contributing a lump sum each Jan.

If B then should we:
1) Contribute from regular savings
2) Contribute monthly the year prior to a separate fund and transfer the lump sum in Jan.

C: None of the above

Thanks in advance for all the help!

Mike