Mage/rogue doesn't offer any stealth. Rogue/Mage does, but it also isn't very useful in PvE. I would stay away from r/m and try m/s. You get to have three classes total, and you can switch between different 2-class combos that way OP, so you could pick mage, priest and scout, and then play m/p sometimes and m/s sometimes, as well as p/s and p/m.
M/P is arguably the easiest class combo in the game to level. I think m/p is the best choice for you to level your mage with, and then you can pick up scout as soon as you hit 20 on your mage. Then level you priest on p/s, which is the easiest priest combo to level. Level the scout in Xaviera, play it as s/m and use your mage gears and just fireball stuff. If you choose m/p/s, you only need to get the scout to level 50, cause it will always just be a second class for both priest and mage.
M/P/S is pretty darned easy to gear and level, cause unless you want to play your scout (and why bother?) you can just use one set of gear on all classes.
M/P is good at endgame, M/S is good at endgame, P/S is superb at PvP. Win win win.
As a mage, you are already ranged. M/S has nifty ranged fire arrows too. You won't get stealth or melee from m/p/s, but it's pretty darned versatile, and no mage really wants to melee, and no mage will ever have stealth anyway.