I'm personally going with Natsu/Lucy, the main reason being im a fanbo-er, i mean the fact that any eventual relation would be calmer than whatever Erza and Natsu could have.
With all the things that have happened in such a short time, it seems reasonable to expect even more and worse things to come.
Really, both of them need some calmness in their life, not more battles and fights and competitions and whatever else Erza and Natsu will do.
Besides, you would think Lucy owed him something for all the help he has given her when it comes to getting all those keys...
Maybe thats just me though.
Anyway, my main argument has been said, but I'll repeat it just in case:
the reason Lucy/Natsu should go together is for calmness, and so poor Natsu doesn't get a heart attack from all the times Erza scares him.