Definitely do a one way.........having a return ticket has actually hurt a few people due to airlines 'accidentally' reporting that the beneficiary actually left on their return flight (when they didnt) which has caused HUGE headaches when it came to their next step with USCIS. Both cases I saw were K1s where when it came to AOS, they were denied as the beneficiary 'left' before AOS was approved (when they didnt....the airline just said they did). USCIS's stance was 'prove that you never' which they couldnt- their physical presence in the States at the time didnt prove that they didnt leave as according to USCIS the 'could' have left on the flight and then got another flight back to N America the next day and entered the US without inspection. Ludicrous I know, but it took a lot of work and battling with the airline and US by these poor couple to get it sorted (and these are just 2 cases that I accidentally came across- its possible that this could have happened to other also).
So definitely go with a one way to be on the safe side.......you dont want to find out when it comes to ROC that the airline accidentally said you left 2 years ago and are still out of the country with out a re-entry permit (therefore possibly abandoning GC).
Extreme case scenario I know, but I always like to reduce any possible chance of potential hurdles when it comes to USCIS.