My experience is mostly in line with Hoss'. In a nutshell, the Stag is a glidier Eagle X (sizable turn with a come back fade at the end) and the Opto Saint Pro is a faster Teebird. The Saint Pro is beefier than the Stag by a good margin.
When I threw ~300', the Stag was -1/3 HSS/LSS: a hair of turn and a reliable fade. Now that I can push 350' with them, the Stag is more -2/2: from an 8 o clock hyzer release it'll sit up, turn out to 250'-275', then fade back to finish roughly straight ahead. I can flip them to flat from a deeper hyzer, but they have a lot of turn to them, regardless of weight and plastic. They're junk in the wind.
The Opto Saint Pros I've thrown (ranging 173 to 176) all fly straight on a flat, full power throw out to ~350'. I like to throw them on a slight hyzer, on which they'll sit up flat and cruise before the nice, reliable fade kicks in. They'll turn in a strong headwind, usually fading back to finish straight. With varying release angles I'm comfortable throwing them in a lot of windy conditions, especially if I can ease up on the power a bit.
The Gold Line Saint Pros I've thrown lie somewhere in between the two. They've definitely some turn and a later fade, though not nearly as much as the Stags. They're are a bit more dodgy in the wind. I'd put them at -1/3 HSS/LSS at my power. At present they'll fight out of an anny a little too early to make them as good at shaping lines at the Stag, but I imagine that'll change once they wear some. Right now all my Saint Pros are pretty fresh.
Personally I find the Saint Pro to be a notch faster than the Stag, and significantly more so than the Teebird. It feels like it's edging up into control driver territory, whereas the Stag is definitely a fairway. While the Stag has developed too much turn for me to provide a reliable fade when I need it, it's a great line shaper, and probably nets me the easiest distance of any fairway I've thrown, due to the combination of sick glide and lots of turn plus the finishing fade.
The two molds pair really well for me. Similar feel in the hand. Both are really good at what they do. The Stag gets a lot of hate because of how off the numbers are, especially the 0 HSS, but if you approach it looking for a stable line shaper it's hard to beat in its speed class. I might try to beat in a Tournament Stag to see if it can kick out my Rivers. It's been a while since I threw a worn Stag but they really start to turn after some wear.