Borderlands bundle - Is it worth buying the 2nd tier?

Depends how far you want the game. Would you buy Borderlands 2 if it was that price on Steam? If yes, buy.

but it's not that urgent

No. Wait for the next sales or win the game on raffles.

Better buy maximum. Borderlands 2 GOTY is more content, a lot, and all DLC's are Awesome

For future - if you think you may want to buy bundle laters, but don't have enough funds and you're afraid price will rise - buy lowest tier possible - in some bundles it will even let you buy for 0.5$ or 0.01$ (these two don't give any keys, but it's not the matter in this case) - you can later on add additional funds to your purchase to reach tiers for the price they were in moment of your purchase, not for price they are now ;) So in your case you'd be able to buy this tier 2 for 4-5$ as it was before instead of 6.7$ :>

I'd bet all the money and buy new Borderlands =P

nope. unless you want borderlands 1 you better get the complete version at 10-12$

I already had Borderlands 2 and 1 but I bought the second tier for the DLC.. I should have gotten then highest tier. I was a bit agitated to find that it was a single key, so I technically wasted a base game key inadvertently.. but the DLC certainly has improved parts of the game.

I'm scared that you wont read this anyways but.
Dont buy it. Wait for a Steam sale for Borderlands 2 with the GOTY Edition & all DLC's. For 1 Dollar you cant go wrong with BL1 but.. the rest is meh...

If you really like borderlands i think 15 usd for everything is a very good price. If you are unsure if you want to "waste" that much money you can buy tier 1 for USD and try borderlands. borderlands 2 is better then the first but at its core is very similar to the first game.