one thing to bear in mind is that if your cooler is WAY too big for the batch you're mashing, the small thermal mass and large head space is going to make it cool off faster so it may not maintain temps very well if your cooler insulation isn't great. partial mashes with 5ish pounds of grain are going to be WAY undersized for a 10 gallon cooler.
If you're going to be doing AG batches in the near future and are thinking of doing big beers and/or 10 gallon batches soon, then the 10 gallon cooler isn't a bad idea, but it really doesn't hurt to use a more appropriately sized cooler in the mean time, ESPECIALLY if you can swap the valve/manifold hardware into a bigger cooler later.
I started AG with a 5 gallon rubbermaid and used it for quite a while before getting a 60qt ice cube for 10g batches. The valve/braid swapped directly into the new one, so I can use it on either one - so I keep the 5g around for smaller batches.