I'm not positive on the rule set for xp sharing, but assume it follows the normal group leveling span, which is:
Highest = 1.5 * Lowest + 1 round fractional results down
Lowest = (Highest - 1) * 2/3 round fractional results up
Leveling Chart Personally, I'd go with a tank merc (make sure you put it on Main Tank, right-click name in group window, select Roles - Main Tank) as they're not very good at aggro til around 20 or so, but you shouldn't need to use any high aggro spells till much higher (i.e. avoid slows and debuffs at low levels, add in memblur line whenever you get it, if you're still drawing aggro from nukes and dots, where they don't really do much other than add a ton of aggro, just keep to nukes and dots, and keeping the tank buffed with haste, procs, and DS). A tank merc can basically solo your toon to the low 60s without any help from you, other than pulling the mobs. As an Enc, you can probably keep the tank till the 70s if need be, but it's probably better to use charm and a healer merc once in the 60s for molo, or use the higher level toon to fellowship xp share.
Whatever way, simply getting to the 50s, I'd go with a tank merc, they slaugher reds till then without any help. Usually the only thing that really slows a tank merc down is dots at the lower levels which pretty much counteracts their innate regen. Using Taper Enchantment to remove the dots, or avoiding dotting mobs help alot until the merc gets higher level where it's innate regen out does the dots damage and you can ignore it.
I used to go with healer mercs at lower levels for melee / hybrids, but after trying a tank merc with a War, it's the way to go. I only add a healer merc at low levels if I'm boxing and already have a tank merc, but even then I generally go with a dps merc until the 50s as the second merc, since the tank won't really need any healing till then, and as long as the real toons aren't taking aggro for long periods, you don't really need any heals, especially with regen potions and the occasional heal pot.
The tank's aggro problems at lower levels will probably be good in practicing how to control aggro versus a real player twinked tank. It was alot of fun with a real Ranger and Rogue, the Rng would have to range for a bit after pulls, and the Rog would have to evade alot at low levels. I used to think it was a major drawback of the tank mercs, but since it only lasts till about the low 20s, it's not so bad, and helps build a good routine for real groups, and keep skills up that you wouldn't work on solo.
Hope this helps,
Yther Ore.
Edited, Jul 14th 2011 7:58pm by Yther