For weight, you're basically duplicating an Ironman; the tubing differences (heavier Infinity fork, lighter Prestige frame, but more of it due to relaxed geometry) are a wash. The Smoothie is less aggressive, not intended to be a competition bike. Closer would be the Stanyan. Both are better century frames than the IM, until the pace picks up; then you'll be working harder, no doubt about it.

They are simply different bikes.

The Soma Stanyan is comparable to a Centurion Semi Pro from the early 80's.
The Soma Smoothie could roughly be compared to an Elite RS or a later Lemans RS, framewise.

I'm sure it's easier to find the newer frames, and I really like the Stanyan.
Not for $600, but I really like it.