You get what you pay for. The Soma rush is a great frame, and has a standing reputation as so. 183rd st frame? Never heard of it. Is it a new company?

For my second build, I was heavily considering a Soma rush. However, since this was going to be a street build, i.e., a bike that would get plenty of abuse, and would get locked out occasionally, being at risk for theft, I figured I might as well just get a cheapo frame. Ended up getting a Pake', which cost me about half of what the Rush would have.

Get a bike that fits you and works well for you within your budget. Don't get hung up on details like chain tensioners and slightly sloping TT's. But yes, continue to ask around. A lesser known frame, like the 183rd could go either way. Could end up being a shoddy build, or it could end up being a great buy. Do your homework. Try and get as much info as possible on this lesser known brand.