for what you are doing on the iMac even the i3 is already more then overkill ,
office, parallels for develop with .net, some ilife (photo and video) nothing thats even stressing the i3 processor neither will it stress the graphics card , why does everybody thinks to need 5 times the power thats actually needed for the tasks they plan to do and the argument of some "i might in the future need more power eventually" is pointless , as it would be better and more effective then to get a new Mac in the future when you really need more power
for me thats like buying a porsche for the trip to the shops that are a 5 min walking distance away

if you would do serious gaming (crysis and the like),professional filmcutting , animations like the matrix or so , then ok then you might need the power of a i5 and a bit better graphics card , but parallels, ilife ....nothing a 2ghz core 2 duo iMac could not handle in reality .ok the i5 is some milliseconds faster , but are you really sitting in front of your computer with a stopwatch monitoring every task ?
so yes i agree with the apple store staff that you will not even notice any kind of performance gain even with a stopwatch ,
so save your money get the i3 and spend the money you saved on something useful for your needs , a ssd , a bit more ram, or a holiday....just for example
and in 5 years from now will do exactly the same tasks at the same speed as its doing things now, and it will handle OSX lion too once its available