I've purchased a 335i when it first came out back in 07, and now leases an A5.
Audis are much more expense to lease. Residuals is around 54% vs 60% for the Bimmer on 36month leases. Also stocks on the A5 is more limited than 3 series coupe, and most dealers will not give too much discounts. A5 is a low-volume car, 3 series coupe is not.
Driving-wise, it is way down on power compared to the 335i, but handling is actually great with AWD and 19 inch summer 255 tires, it pulls more Gs in the slaloms than the heavier S5 and the 335i. With the 335i, traction control always kicked in whenever the ground was wet due to having too much power and RWD. Looks-wise, it is much more of head turner than 3 series, my guess is due to a combination of low volume and the sculptured styling. Interior is also more modern/sleek than the 3 series.
A 3 series will be better "value" to lease, but if you want to lease the A5, lease the Premium Plus trim instead of the Prestige, because the Premium Plus will have 2% more residuals over the lease period. Order an A5 Premium Plus then add any additional options individually.
If you want to get an S5, then I suggest you wait for the newer supercharged V6 (currently in the S4) instead of the outdated V8 that weighs down the car. The only thing the V8 is good for is the exhaust note, everything else is better with the supercharged V6 including fuel economy.