While there is no "best" in this subjective hobby, I have owned the SimAudio I-5, and recently purchased a 100B-SST w/ DAC (3 months ago). To my ears, the 100B-SST has better bass control, resolution, imaging and neutrality than the I-5. The DAC is also substantially better than the cheap transport DAC (a Pioneer DVD) that is used to feed it. It's a damn fine integrated.
I also paid significally more for the 100B-SST than the I-5 so it should be an improvement. Again, unfortunately, this is not necessarily true in this hobby. The I-5 is still a great little integrated, and for a used price of $1400 or so it is tough to beat.
It should be noted I use(d) Totem Model-1 Sigs with both the I-5 and 100B-SST. Totem speakers generally perform poorly when underpowered; and they seem to be power hungry.
My local dealer carries both SimAudio and Bryston. Maybe your does too? Listen and see what you like best. You might also want to check out the recently updated Bryston 60-SST (the same output devices to the entire SST amplifer line) for a more "reasonable" comparison to the I-3.
I have not done an I-3 to 100B-SST comparison, but based on the I-5, I would expect similiar but even larger differences.
