best size commuting tire?

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I only have one bike that I use to train and would like to start commuting on it. It's an old Schwinn, nothing fancy or fast. Anyway, is there anything wrong with commuting on 23's or 25's?

I've only had 23's, how much of a difference/loss with 25's will there be on training rides? I'm not too concerned yet with extra weight type stuff as I need more work on my engine. But if there's a significant slow down with 25's I'd like to know.

And changing tires all the time is probably not a good option. Maybe a best bang for the buck affordable racing wheelset (<$200) might be an option.