Mash Tun Question

After about a year of brewing extract and lurking on this forum for a couple of months I'm finally ready to give all grain a try. I'm having trouble deciding on my mash tun setup though. I plan on using a converted cooler as my mash tun, but I don't know what size to get. Right now I brew 5 gallon batches, and plan to do so for the foreseeable future. However, I would like to be able to scale it up to 10 gallon batches in the future. Is there any problem with mashing for 5 gallon batches in a 10 gallon setup? I like doing high OG beers and am afraid there won't be room for all the grain in the smaller cooler for a gallon batch. Right now I'm leaning towards the 10 gallon, the only drawbacks are the extra cost and space issues. Also, I'm probably going to be lazy and buy a pre-made converted cooler mash/sparge setup from Morebeer:
5 gallon - http://morebeer.com/view_product/15627/102307
10 gallon - http://morebeer.com/view_product/15631/102307
Any thoughts on the matter would be appreciated. Thanks.

I do it (hehe, I said do it) in a 10 gallon no problem...5 gallon batches that is.

I wouldn't dare buy from them, build them yourself, so much cheaper

Go with the 10 gallon cooler MLT. The 5 gallon isn't big enough for big 5 gallon batches. I've used a 10 gallon MLT for all of my 5 gallon brews and it has worked great.
Let us know what you get!

You don't have to use a round cooler if you are batch sparging. The 48-52 quart Igloo coolers work pretty well.