You need to think about the following before making a decision;
1. What's your average monthly use?
2. What's the variance on your monthly use? Does it spike up/down at certain times of the year - seasons, vacations, family events etc.
3. How much you really need/want the US roaming option.
You can glean most of the answers from you bills for the last year, and ask yourself if your needs are going to significantly change in this/next year.
Overages and expecially US roaming are expensive on Rogers, but if they're occasional-only and your average monthly use falls close to the 40min cap, then the $10/mo plan might suit you fine.
However, if there's a lot of variance - 10mins one month, 50mins the next, and not a lot of roaming, then Paygo will probably save you money.
Finally, there are alternatives to using Rogers roaming in the US if your usage is more than occasional. In addition to prepaid SIMs as low as $10cents/min (including LD), WalMart and other retailers sell prepaid phones/cards from TracFone, Net-10 and others. Some of the phones are less than $20 and include some airtime. As with the CDN plans, higher value top-up cards have 1 year expiry.