First of all, SSG doesn't do true IDP, it does deep inspection ( again watered down sub set of the entire signature set found on IDP)
As far as performance, the SRX are cheaper and does firewall in addition to IDP.
Low end IDP 75 for example states it can do 150mbps IPS ; IDP 250 up to 300mbps , IDP 800 1 GBps , IDP 8200 10GBps,
SRX 210 can do 80 Mbps , SRX 250 can do 250 Mbbps , SRX 650 can do 900mbps.
However the standalone IDP also has some advance features not available on the SRX ( application volume tracking, honeynet, application policy enforcement, backdoor..)
My recommendation is if you dont' have a need(regulatory,compliance, advance features of IDP) for a dedicated IPS/IDP box or and don't need 1gb~10gb of IPS inspection test out an SRX as it could be a good fit with firewall / vpn capabilities.
Joe