Well, look at it like this:
Recruit packs: Great for getting consumables
Veteran packs: Good for building up your mod/weapon collection. Chance to score class/race combos as well (class/race combos are marked as uncommon in the full game, instead of rare like in the demo)
Spectre packs: Best possible chance at getting the rare goodies. I've gotten some special items out of these, like a permanent increase to the amount of medigel I can carry into missions, as well as some pretty snazzy weapons. You can also pick up multiple rare items out of these (I got the Carnifex and Revenant from the same Spectre pack). As mentioned in a previous post I made, there's an additional quality tier beyond rare, and Spectre packs are your best chance at grabbing those.