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.
If you & your mates want a stable platform go for a 1.5.2 pack, the 1.6 mods are mostly not stable "dev builds" or unavailable in some cases, some mods are aiming for 1.7 atm & omitting 1.6.2 as such.
As bobs said above if you wish to wait for the latest versions of each mod you will never get to play, so the best bet would be to use the pack suits you guys best for NOW, in a month or two, if you guys wish to update to a newer version pack it will require a world reset, but that said often a month is enough time to learn the new things & make all items/machines/setups, so going forward to a new pack at that point will arm you & your friends with far more knowledge, aint that what sandbox games are disposable games ?, build it all, delete, rinse & repeat.
I got tired of waiting & so, apparently, did the FTB team, so now I'm enjoying unleashed. Getting vanilla hoppers into my sorting & processing builds was what I really wanted, anyway. 1.6 seems to have made some major changes to how the software runs behind the scenes, requiring many mod developers to completely rewrite their mods. You can't blame them if it takes longer than expected & you can't rush them if you want playable results. I know nothing about programming, but I do watch Direwolf20's Let's Play server series.
In all seriousness, some mods seem to be skipping 1.6. If those are mods you absolutely can't play without, then skip 1.6. If not, try out some of the 1.6 mod packs which have already been developed, either on ATLauncher or on the T**kit launcher, if you are so desperate for a 1.6 mod pack. I'd like to think I've got a decent basic pack, but hey... if I didn't think it was decent, I wouldn't have published it in the first place. I'm still trying to get ExtraUtils to fix the bug with NEI rendering of the ender-thermic pump, so it's still not included yet.
Eh... I suppose it's kind of a 'put up or shut up' kind of thing. There -are- 1.6 mod pack out there. They just aren't FTB mod packs.
Personally, I'm tinkering around in 1.5 and waiting for 1.6 to arrive with FTB. However, upon arrival of 1.7, I will very possibly lock down and really get to work barring something substantial that is as yet unannounced.
Starting to feel like Mojang is working really hard to have Mod API for MC 2.0, which would be the epitome of awesome sauce.