SBI
As a pilot that recently passed both days...I feel like if you're a current 121 Captain, there's no need to spend $$$ on the job prep sites. Unless you need the "confidence booster". Out of 11, I'm the only one I know of that didn't spend for a job prep. and it worked out. But then again I've been prepping for well over 8-9 years, with general studying of gouge and what friends have experienced recently. Military folks might need the prep if they've been out of CRM or actual cockpit time in recent years. But having flown with many ex F-18, A-10, KC-135, P-3, C-2, E-6 First Officers over the past few years at my current job, all of them are solid and learn quickly!
FYI, the process has just changed...so we've been told. My advice, just be a solid pilot that knows basic job knowledge, AIM/Aerodynamics/Weather/etc. The other parts are just there to see if you're a good fit, IMO. Above all else, SAFETY is the key and should be emphasized at all times, whether panel interview or SBI. (like we've been trained since day one of flight training)
Good luck to all! This is an amazing company! I was truly impressed at the money spent on facilities for crew and staff. Top notch is not saying enough!