Rat eaters do get bigger quicker (in my experience). But if fed appropriately they don't necessarily stay bigger.
Mouse eaters will need to eat more mice to maintain and gain weight than rat eaters. If you don't mind feeding 3-8 mice per snake, per feeding they should do alright. If this isn't your thing, switch them over to rats now. The sooner you do it the easier it is.
As far as one being better for fertility than the other the only argument I've heard about this is that the rats have more calcium in them, but if you are feeding enough mice I would think it would balance out.
It is usually more economical to feed rats though. My local pet shop sells adult mice for about $2 a piece and small rats for about $4 a piece.
One small rat is about equal to three adult mice.
Two small rats at $8 or six adult mice at $12. The difference can add up fast if you aren't breeding your own.
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It's actually not that I love or even like snakes. I just hate rodents.