I have an 18i20 interface, but I;m trying to figure out the best way to go about mic'ing drums with it so I can have fuill control over each mic with Splat.
 
Would it be best to invest in the Octapre or a mixer? If a mixer would be best is there an affordable choice that will give full control in SPLAT?

If you use an ADAT AD/DA Converter/preamp with the 18i20, each ADAT input (and/or output, if using bi-directional ADAT) is fully addressable as a separate I/O port in Sonar.
 
For a mixer to provide the same functionality, it's either going to need its own ADAT output to your Scarlett interface, or it would likely need to be installed as it's own ASIO-capable audio interface device - which would likely cause a lot of problems if trying to use simultaneously with the Scarlett.
 
FWIW, while the Octapre is probably a bit "better", the Behringer ADA8200 works great for ADAT I/O, and is several hundred $ cheaper than many other brands...

Interesting, thanks for the info.

I agree above. I also have the 18i20 2nd gen, and do far it's been great with SPLAT.
After I got to know the Focusrite I/O software I use it for almost all listening and recording in my home studio.
😃

So your interface has 8 XLR inputs. I don't see an issue that's certainly enough to record drums.
And if you desire more XLR inputs you could buy a small mixer.
As far as stand alone mixer interfaces go check out the Soundcraft Signature 12 MTK 
this is a capable 12-14 channel audio interface , one of the reviewers was a Sonar user and claimed no issues using it with Sonar. 
 https://www.sweetwater.com/store/detail/Sig12MT