Compare to connect IP PBX with PSTN Vs SIP Trunk, indeed it save money, you would replace all of them or just part of it, I mean half PSTN and half go for SIP Trunk ?
Any existing Asterisk IP PBX Users already switched to SIP Trunk ?
I think that for the most part PSTN is still more reliable than SIP. SIP has more points of failure. I would suggest to start using SIP a bit, see what issues come up and gradually move over to SIP.