Both answer your phone with a real voice. Lobby keeps the per-call math lower: Starter includes 100 answered calls at $95/mo, while Smith.ai's public self-service pricing shows about 50 calls at the same monthly price.
Lobby answers on the first ring in a natural voice, understands what the caller needs, books it or warm-transfers to you, and captures every lead. More answered calls only matter if each one feels handled.
A missed call is usually a lost customer - more than 6 in 10 who reach voicemail never call back. Lower per-call cost means you can afford to answer more of them.