1.5 won't be compatible with 1.6 due to change with fluid control in forge so you will need to make new world in 1.6.
In 1.6 from main mods aren't "on the market" Thermal Expansion - aqueous acumulator, redstone energy cell and tesseracts are needed like oxygen for breathing, Thaumcraft - right now in dev version in Forgecraft testing server, Ars Magica 2 - also great magic mod - official MFR version - there's port to 1.6.2 but right now there aren't Power Converter and Nether Ores - those mods are very helpful.
Vanilla changes which are in 1.6.x version are wicked - more agressive zombies - especially when you cave and horde of zombies in gold armor attacks you - sweat
, coal block, different charcoal texture, horses which are fast transportation, mules with chests - new ideas to massive buildings like stable etc. Carpets are good visual material to make buildings look good. Also hardened clay which can be coloured looks very good.
1.6 is worth playing right now - even if Thermal Expansion isn't right now avaible - but eventually will be.
I'm starting to use Ender IO instead of waiting for TE - it have aquous acaumaltor, liquidicts, redstone conduits, redstone cell and even MJ solar panel - of course other names and different recipes. Also for transportation of fluids and MJ eenrgy i will be using teleport pipes - 1 pipe uses 8 diamonds but faster i will find/ produce/ bee produce million of diamonds instead of waiting for TE
And if in FTB there won't be official 1.6.3 pack - yes 1.6.2 will be changed in 1.6.3 due to bug with existing spawners, nether fortresses which where generated in previous MC versions - you have to make yourself modded pack.
Check out
http://bot.notenoughmods.com/1.6.2.html and combine your modded pack.