Well, in that case I would just pony up the extra dough for the 10-day MYW pass. The lowest price including taxes we've been able to get on a child's 7-day PHP has been $252. The advance purchase price including taxes on a fully loaded child's 10-day MYW is $360.
If you divide the extra cost across the 3 additional park days you get, you are paying only an average of $36 per day. Compare this to $51 for a single day ticket without park hopping or an average of $57 per day for a 3-day pass with park hopping under the new system. In addition, you'll be getting an extra "plus" and you'll be able to use Disney Quest.
Another comparison would be to purchasing a 7-day PHP and a 3-day PH. I found a 3-day PH on eBay with a "buy it now" price of $165.
So, getting back to your original question, if it will be a little while before you are going to be in WDW, you might wait a bit and see if the online brokers lower their prices to purge their inventories. If you'll be going soon and want to get everything done, I would recommend buying the 10-day MYW passes.