Carrots, as they would hopefully be high enough to avoid the carrot fly?
But I would say itwould be a waste to use a whole bag-depth of either topsoil or manure. I would empty one of them (use it onyour plot wherever you need it) ,half fill it with old soil or whatever you've got around (I used clumps of dug-out grass inthe bottomof a dustbin last year) then the good stuff, thenplant. Then do the same with the other one. Just my idea, I'm sure others will be along soon!