Borderlands – Farming Varkid morph pods

Not sure which I would recommend personally; you can freely run around Tundra Express to find all the Varkid pods (TE and Caustic Caverns are the only places with Varkids so I assume that's what you meant, not Arid Nexus), while Caustic Caverns throws many more tough enemies between sets of Varkids. I do believe there are less varkids overall in Caustic Caverns, but there are two clusters with great numbers of them (at the very start, and midway into the level after the first set of Treshers and before the Spiderants).

However Caustic Caverns spawns you closer to the first set of Varkids so you could more easily farm, quit, rejoin, then farm the respawned varkids and repeat, plus they should be of a higher level. As a bonus, the Chubby Varkid sometimes spawns at the start of the Caverns so you might get some decent loot out of it as well.

Also note that unless you've finished the game again in TVHM mode the enemies won't scale, so you're probably better off in Caustic Caverns either way as long as the enemies levels are higher (though if the enemy is at your level or higher, level distance no longer matters and you'll get full XP from that enemy).

However as far as "recommended", I really wouldn't recommend grinding levels unless you've actually hit a brick wall (we sure did at the boss of the Wildlife Exploitation Preserve), since by the end of the game you're almost sure to be level 49/50 (I've see 3 people, myself included, reach 50 during/after the final fight) even without too many side missions. Grinding should really be reserved for level 50 loot unless you feel you need it; keep up normal progression and you'll level well enough, the enemy levels/XP balancing is quite well designed (if a bit difficult) for even playthrough 2.