I went with the Igloo 10g plus bulkhead kit as I do either 5 gallon batches of beer (mostly around the 1.050 to 1.070 mark)
for $60 total.
I loose maybe 2 degrees an hour when mashing but then again I mash in temperatures around 68 degrees year round so I don't have that big of a temperature delta as someone from say, Minnesota or Maine does.
Edit: Just re-read your post saying you're using a 70q. That 10g might be a little too small for you. I just visited the Igloo page and saw these three coolers. If they can hold ice for 5, 5, and 7 days respectively, they should retain heat well too.
edit#2: $350?!!? nevermind......