You can never say that one type of setup is better than the other because they are designed for different purposes.

Twin turbos give excellent low-end response, but the turbos need to be small, therefore not as good top end.

You need a larger turbo than you can use with a twin setup (unless its a V8) to make really big top end power.

Twin sequential turbos give the best of both worlds with one little turbo for low-end and a big turbo for top-end. But like YRL0SS explained they are impracticle for the backyard builder.

And, yes, turbos ARE designed to work at specific engine RPM. For example, the RX7 we are building at the moment will have a TO4S BB turbo that will give a power band from 3000 - 8000 rpm. This is the deciding factor when designing the port shape for the 13B.

But lets not stray from the original question. I reckon you should get an S15 T28.