What I mean is what would be a more believable Guerrilla warrior for an elven army? A beguiler, a master of Illusion and enchantment (great Infiltrator) or a beguiler/Avenger (great assassin)...

With a Beguiler, my combat potential is reduced to Whelm, Buffing the party with Haste and Great Hope, or avoiding combat all together with Illusions, Ghost sound, invisibility, and silence. The damage potential for my character would be limited to the 5d6 provided by the whelm spell, 1d10 from a heavy crossbow, and Save or Suck spells (provided that they do not save)

With an Avenger, my combat potential is greatly increased with Sneak Attack (Now allowing Ambush Feats), Death Attack (Great for a Single Sniper or taking out a sentry, because of the high Intelligence makes for a high Save DC) and the ability to use poison without a risk to myself (I know that poisons are not cost effective but are great for fluff, and for spending excess gold), An extra Attack (+6/+1 BAB), I may have less Illusion and Enchantment Abilities but with Practiced Spell caster and a little ingenuity I can use lesser spells to secure the ambush... I know that a full beguiler could also secure the ambush but would have less to do during combat...

The fluff I have been talking about is... My character is an EX-Soldier that was part of an infamous unit of guerilla fighters that threw the conventional Geneva Convention out the window, the war that he fought in was decades ago yet is still bitter about about his side's loss. Now he is a contract mercenary, living off of adventures and their need for someone of his skill set.

I wanted to know if a full Beguiler or a Beguiler/Avenger Mix would better fit my established background story.

I am not sure about the feats I would take or how I would distribute skills but I do know I would like to throw some points in disable device, Craft (Trapmaking), Use Rope, and perhaps Craft (Alchemy) (not sure if it would be alchemy) for his knowledge of poisons... As for Feats I know for a fact one of the feats that I would take would be Darkstalker from the Lord's of Madness as, when combined with "Magic Aura" (it can take away auras as well as add them), it will allow me to sneak past guards put in place to detect invisible or otherwise magical infiltrators...