I love the Pan Pacific so it would almost always be my preference unless there was a huge difference in rates or status/upgrades elsewhere. I have status within the Discovery program, which is another tiebreaker in favor of the Pan Pacific for me personally. The Pan Pacific is usually very good about upgrades when booking direct, though I typically book a Pacific Harbour Studio outright just to make sure I get it.
I think the big thing is how important club access is to you. The Pan Pacific club level is nice but doesn't offer a ton and in all honesty, there is no reason to spend time in a club room when you're in Singapore and can throw a rock any direction and hit incredible food and drink for a couple SGD. I always think I'm going to stop into the club for a drink and some snacks before dinner, but I rarely end up doing it. I don't bother with free breakfasts in Singapore either, I have a finite amount of stomach room that can't be wasted on a western breakfast.
Everything else is about equal in terms of location, distance to MRT, distance to sights, etc. MO water views will be almost entirely unobstructed but those high floor rooms at the Pan Pacific are high enough to have a pretty great view, including all the way out to the Strait.
I think if the price is equal, you can't really go wrong. I know that isn't particularly helpful but if comfort and convenience are your deciding factors, they're almost identical.