I love my E3's. There is a big difference between E2 and E3, and well worth the price increase. I could not justify paying three times the amount for E5 over E3.

I do a lot of sound work for bands (as a hobby, not professionally) and have not bought 6 sets of E3 for in-ear monitors. We use them for keys, drums, bass, accoustic guitarists and they all love them. Even the bass players are happy with the low-end response, which is where the E5's outperform the E3's. 6 sets of E3's for under US$800 was managable, 6 sets of E5's at more like US$2700 was not. I bought mine in two lots of 3 off eBay.

The E5's are also more sensitive, at 122dB compared to the E3 at 115dB. But even the E3's 115dB is way more than most over-ear headphones. I have never managed to find the specs on the Bose QC1/2, but expect they are more like 102-105dB. The extra sensitivity of the E5 makes them seem "beefier" at the same output setting on your audio source.

The isolation will be the same for E3 and E5, especially when using custom molds.

The E5's have the "memory fit cable" which helps to hold them in place. This may be helpful if wearing them while sleeping. The E3 cable is much improved over the E1's which they replace. The cable on the E1 was its biggest problem.

So for my money (and it was my own money), the E3 was best value. But if you can affort the extra, the E5 is certainly a very good product.