To change or not to change, that is the question?

Well yesterday at Newington bike I decided to change my rear cog set out to 11-28 ultegra, but I have an old Ultegra SL 6600 unit that has a max of 12-27. Performance bike has a dura-ace 9700 on sale for 161 and change that would work, but do I want to spend the benjamins, yes, but I do not have the $ yet. Hmmm...what to do, what to do?