[Patrick]

Nothing wrong with running a 3s in series with 2s, if the cells are the same type and usage. That was my main setup (also using the 1515 2.5d) for about a year, good setup.
I'd say 2 packs in series is usually better than 2 packs in parallel.
Unless you want parallel packs for huge runtime the there won't necessarily be much difference in weight and performance. A 3s + 2s 5000mah and 2 x 5s 2500 mah of the same C rating and quality should be similar.
The main reasons I think the series setup is better is because it's easier to charge and balance the 2s and 3s as one pack. To balance and charge the parallel packs as 1 you would need a 10s charging setup, or make a harness to link the balance taps together in parallel, and I would avoid doing that.
If 1 packs comes unplugged for what ever reason in the series setup the car will just stop. Annoying, but at least everything will be ok once you plug it back in. If one pack comes unplugged in the parallel setup, the car will run of the pack still plugged in. Since you will probably choose lower capacity (and therefore lower amp output), 1 pack will not be able to handle all the load by itself and could be damaged if you kept driving (might not notice drop in performance in low traction areas).
Only reasons I would choose 2 5s packs in parallel would be if they had a good special on them, or you needed a certain size pack to fit in your car (most 2500mah packs will be shorter than 5000mah packs), or if you had something else that you used the packs in.