Percentage increase > Additive increase. At the current gear level, there's really nothing that beats Expertise because it has such a soft DR curve. If you could get to like 2000+ Expertise you'd probably be better of going Power, but as it stands, the difference between 1200 Expertise and 1300 Expertise is ~2% damage on all abilities, whereas 100 Power gets you +23 damage on all attacks.
So if your attack does 2500 damage on a standard hit, then:
2500 x 1.02 = 2550
2500 + 23 = 2532
So even at higher than 1200, still worth it. Additive increases are better for small damage numbers like DoTs, percentage increases are better for high damage numbers like your class' big hitter (Railshot, Back Blast, Death Field, Heat Seeker Missile, etc.).
That's how I look at it, but obviously you need a decent starting point for the percentage increase to be worth it, but even with 1300+ expertise you have plenty of room to stack Power to get your base damage high up there.
This explains it pretty well for the majority of classes. Yes, some classes will benefit from power more, especially if low, but overall expertise now has an easy curve that makes investing in it a good idea.
Also of note is that even if you are pure dps, the expertise not only increases your damage by percentage instead of a lower flat increase, but it also allows you to mitigate more damage which will invariably increase your dps even more since your up time is longer -- a win/win.
Overall, do the math to see if dropping 25-50 expertise on a piece for a bit more end/main stat is worth it for you, but as a rule, have no fear to go high expertise... it's worth it.