With the gem Adam wins. Not because of soul suck (I doubt it would work on Alan) but because his karmic blast was only accessible with the gem. It messes with concentration, and was never countered even by telepaths. Lack of concentration is fatal to a GL. It's how Adam got at least 1 of his wins against Quasar (Wendell).
Without gem I favor Alan. More raw power. Warlock is tricky as hell, but Alan's also as experienced as they come.
It failed against Surfer one time because of his past experiences with Mephisto. But I'm pretty sure he's the only one and the gem was working independently rather than with Adam at the time so who knows how that factored into it.