Hi,
IMO, the best option is the non-reth interfaces with dual ISP known as Mixed mode:
http://chimera.labs.oreilly.com/books/1234000001633/ch07.html#mixed_mode
WAN interfaces will be individually active on each SRX and LAN interface can be a reth [Active/Standby].
For BGP with the border routers the options are:
1. iBGP [Full-Mesh or RR] - would require an IGP running between the border routers and SRX WAN interfaces to provide reachability of BGP peer interfaces [loopback].
2. eBGP between border router and SRX WAN [Could use private ASNs on SRX]
Cheers,
Ashvin