Go with the air pet. While earth is a slightly better tank, it has about 40% of the agro that the air pet does (even rage swords only do so much).
It kind of depends what level you're at... 'to max level' is very vague... If just starting out, (assuming gold account) a J1 tank merc with a water pet is the way to go, til about 60-65. After that, if purely going for fastest xp, I'd promote a fully equipped water pet and healer merc, and taking on mobs that pet can handle. Fully equipping your pet (that is, tradable gear, NOT summoned junk) will increase your water pet's DPS by 30-70% and it will tank as well as an equivalent air pet. Depending on level of pet (NOT spell casting level, but what the pet's actual level is after summoned) depends on what gear you can obtain and give to it. Defiant at lower levels, the T1 tradable stuff at higher levels - there is a lot of level 92 and 98 gear that is tradable that pet can use. Some 95 required TS jewelry and masks are much better than summoned gear, Fireshard Ring, etc - just remember you need to give pet a haste belt (daily hotzone quest belts, stock up on em) if give it a TS mask without haste... When pet's 100+ you will want to scribe Equipment of the Ethernere and start making dimensional warrior gear for pet, plus the matching jewelry, belts, masks, etc. Farm lots of steel ingot, and make this stuff for relatively cheap, can make a whole set for just a few k plat. A fully equipped air pet with warrior dimensional plus equivalent non-vis slots will out DPS an out of the box air pet, and out tank an out of the box earth pet... A fully equipped water pet will out tank an out of the box air pet, AND do almost 70% more DPS than an out of the box water pet. Just gotta protect the pet, grind in areas that aren't too risky, barely dark blue mobs, and take them out faster Rules for equipping pet: Items must be tradable, attunable is ok. Pets won't equip no-trade items, including ones that have been attuned, unfortunately. Pet equips highest AC item - if it currently has a 90 ac, 5000 hp belt, it will equip a 91ac 100hp belt. So, whatever you give pet, it must be higher AC than summoned gear (not hard at all - anything almost beats summoned gear) Pet only has 1 wrist slot, and no charm slot. Weapons, pet will equip least delay, I believe, still. Lore and class restrictions do not matter for pets, so, pet can duel wield lore weapons, silk, chain, etc armor. Pets do not benefit from 'worn' effects such as pugilist gloves, unfortunately. NPCs, including pets, do not have a 'soft cap' for AC and other stats... so, what this means, is that equipping pets with GOOD gear is a tremendous help. If just equipping with 1-2 random pieces, per pet, you won't really notice a difference. However, if fully equip a pet, there's a fairly large difference. RoS tradeskill items and materials prices are dropping now, so it may soon be feasible to equip pets with RoS tradeskilled items
I would also recommend NOT scouting out for a high AC back item. Your pet focus on pet earring (ie Enhanced Minion ...) buffs your pet using a back slot item.
EM focus cloaks stink for stats. Zero AC, HP, or saves, and almost no dex or attack. They do get a melee mitigation buff though, but if that's not needed, good tradable cloaks are better DPS I believe. Not parsed in many years.
