My advice would also be to wait.
I decided to wait myself... The whole grind will be bad enough as it is without having to add additional grinding due to having purchased the mk XI set.
I did purchase the 3 pieces of the Assimilated Borg-set (Kinetic Cutter Beam, Omega Torpedo and the Assimilated console), but that is personal preference.
Costs almost twice as much for like 5% better. Its worth waiting if having less than The Best will nag you till the end of time. Its close enough though that I have some characters with 11, and some with 12, and I can't tell the difference without careful parsing. Its more mindset than math at that point.
Personally I prefer the XI ground sets because the helmets look lame.
Then buy the XII set and use the tailor to make an outfit without a helmet.
If you plan on running any ground STFs (or Defera dailies) buying the Mk X ground set to get the instant remodulator as soon as possible doesn't seem like a bad idea, but otherwise low rank sets (especially space ones) are a waste of Dilithium and marks.
The assimilated XI and XII sets are virtually identical except for the shield, but you'll want the MACO resilient shield for it's absorb anyway. Unless you want the sector space speed boost from the assimilated engine, which is great for both the tour and DOFFing, you can get a good XII positron deflector, a combat impulse engine, and a resilient shield with plasma resistance off the exchange for virtually nothing and save the dilithium for later.
On the ground, things are a bit different. Borg don't adapt to melee damage, so a good sword is all you need for Defera, but not all ground STFs are melee-friendly. Make sure you know the map and optional objectives well before you go elite on the ground.