I am not sure which model you are comparing but the 21.5″ models offer 2.5 GHz and 2.7 GHz i5 processors for a difference of 250 quid. The 2.7 GHz option includes a 1 TB system hard disk versus 500 GB for the 2.5 GHz machine. It also uses the ATI Radeon 6770 versus the 6750 graphics. The difference may not be worth it for you but a 500 GB system drive will be a usability constraint for you running FCPX unless you plan to make extensive use of external drives.
I understand the budget is tight but you might also consider the 27″ iMac with the 2.7 GHz i5 processor. It seems a better value in my opinion. When you look at future expansion, having two Thunderbolt ports is nice.
Rick Lang
iMac 27” 2.8GHz i7 16GB