For having played both on P99 (mage 52, neccro 53) and raiding a wee bit, here's my point of view:
Solo (xp): both fare really well although the mage is much more laid back. My mage felt like it leveled much faster than my necro although the necro is an iksar so this is one major difference. I still feel the mage leveled faster despite this.
Solo (item camp, BGH): necro has the upper hand, hands down (pun intended
[You must be logged in to view images. Log in or Register.]). The difference is so staggering, it's not even worth getting into the details.
Group: the mage has a pure DPS role (pets, DS, nukes) with a bit of support (summoned item, mod rod and at later level CotH to summon a replacement for a departing group member). The necro does some dps (pets, loosy nukes (except vs undead) but is first and foremost a support class (root, mez, twich, heal).
Raid: life in raid is boring for the majority of the attendants (except maybe for the pullers) but the exitment from raiding comes from downing a big boss with your mates so it is mostly a matter of doing whatever is sked from you. For a mage it will be mostly to send your pets in (vs non fearing boss) and to summon more mod rods than you can count for other members to use their mana (forget about your nukes, they won't land most of the time). For a necro, mostly sending your pet (same as mage), twiching and DoTing. However, necro DoTs don't stack so a couple of necros is enough to have the whole range off DoTs covered.
Hope this helps.