Bigger is Better. Maximise your bottom time. The tight places aren't going to be that different when comparing a 12l to 15l tank.

i would go the bigger tank , there wont be that many places you go that the difference in tank size would make the difference, i would rather spend another ten minutes down the bottom instaead of coming up early

I have both 12 and 15l tanks.

Its annoying swapping between different sized tanks, perhaps all 15 is ok, or all 12, but both can get on your tits after awhile.

Also, after you have been diving awhile, you tend to stop being such an air pig so you don't need the 15l so much. Conversely you get a lot more exp diving the bigger tank, because you're down for that little bit longer, every time. 

Tank capacity is just one aspect, some people find the 15l quite awkward because its heavier, and rounder, which moves it's COG away from you.

I would go with the 12L. 15L are just big and awkward as others have pointed out.

Been diving since the early 80's and have allways prefered the smaller tank. If you can, borrow one and see if you like the larger size.

As for endurance. You'll find as you gain experience, your air usage will drop dramaticaly. 

not as important  but mayb a point to consider.

15l  as said you'll get a little more bottom time, size isnt really an issue in tight places i've found. i personally own two 12lt and prefer less to haul around out of the water. But if its free i usually pinch my partners old mans 15lt. Main reason is 95% of the time my dive buddie is the miss who has better air usuage  than me comes in handy if i go the 15 and her tank is a 12 we'll usually come up with the same air. meaning one of us doesnt have to call the dive early before going thru a tank.

I used to be a massive air pig, and when hunting can still be one at times. I use 15's all the time, they are heavy and big, but when your in 35m it is nice to have leftovers on the way up. If a buddy has a 12 I tend to have similiar air left at the end.

Go the 15's I say, but only because that is what I did, but you'll find 90% of your bussies will have 12's.

Thanks for all the comments guys, Having only used a 12L I think I'll hire a 15L this wkend and see how it feels before buying.

 The cheapest place I have found is that dive shop on scarb beach rd, mt hawthorn. The 12L is $379 and the 15L is $449

go the extra mate and get the 15l , so much more time down the bottom.

Don't forget to drop a few pounds of lead with a 15