i would just buy a 30inch cinema display and hook it on the mac pro and upgrade
the speed difference wont be really noticable, between the i 7 and your 2 xeon's , in real world usage ,
and with a desktop you always have the possibility to upgrade
with the iMac you pretty much stuck
and for quad core you could shove in 2 higher clocked xeons maybe 3.4 ghz harpertowns socket 771 , max out your ram to full 32gb , and fit a better graphic card
then the iMac with i 7 looks like a tortoise on the back in comparison
the speed difference wont be really noticable, between the i 7 and your 2 xeon's , in real world usage ,
and with a desktop you always have the possibility to upgrade
with the iMac you pretty much stuck
and for quad core you could shove in 2 higher clocked xeons maybe 3.4 ghz harpertowns socket 771 , max out your ram to full 32gb , and fit a better graphic card
then the iMac with i 7 looks like a tortoise on the back in comparison