The Innodb table right now has about 9 million records and is 8GB in size (4GB index size). The memory table has a subset of the same data - 1.2 million records and is 276MB in size.
Another issue with using a second Innodb table would be the load time. Every 3 hours we empty the memory table and load the subset from the innodb table (we actually use an MyIsam table in the middle.. but that's not important). Loading the table takes less than 10 seconds. If we had used an innodb table instead of a memory table, this load time would be significant.