I assume you meant "SSG20", in which case, for all intents and purposes, the SRX100 is more than a "match" for it, purely on performance numbers.
The SRX100 supports significantly higher firewall throughput, higher VPN throughput, and more concurrent sessions. The only spec (on paper) where the SRX100 is lower is the new connections per second.
If you need the different WAN options, the SRX110 adds built-in DSL (POTS or ISDN) and 3G/4G capabilities. If you need different options than that like T1/E1, etc., the SRX210 has more options for Mini-PIM cards.