70-200 F2.8 IS v1

a seller claim to have a F2.8 IS MKI in stock for 1750$.
prices in here(my country):
F4 IS =1430$
F2.8 non IS=1500$
2.8 IS II= 2950$

i did compare the '2.8 IS I' @ F4, to F4 IS @F4 using the-digital-picture tool. i don't see a noticeable difference(maybe its just me?), so i guess i wont miss anything other than the lighter weight of the F4 IS & the price.

my Question:
1.am i making the right decision by going for the 2.8 IS MK1 or i'm missing a point?
2.is the DOF much differ between F4 & F2.8?(light is not an issue for me, i use flashes)

i mostly shoot portrait with a 50D btw.
thanks in advance

*sorry for double posting, dont know how to delete the last one!

if I may ask what aperture setting do you usually shoot with strobes? that might just give you your answer.

Jam.radonc wrote in post #11570664
if I may ask what aperture setting do you usually shoot with strobes? that might just give you your answer.

depends on how much DOF i want. sometimes i lower the power of my flashes and set a faster aperture to narrow the DOF. problem is, i dont own any faster lens than F4, thats why i asked how is the difference between F4 and F2.8?

It's all dependent on your shooting length from your subject. Perhaps this may help http://www.dofmaster.c​om/dofjs.html