Uncollected to Cavalier Help

JC10100

I have just finished Act 5 and wanna grow my roster for act 6. I just bought 2 6* nexus crystals and pulled Prof. X and Shang Chi. I took them Both to R2.
My best top champs are

5* 565 Herc 140 siga
6* R2 Guardian 20 sig
6* R2 Shang Chi
6* R2 Prof. X
5* R4 Void 80 Sig
5* R4 Mr. F
5* R4 Ham
R5 Steve

I have 5* R2 I'Doom, 5* R2 Domino, 5* R2 Warlock, 6* Venom

Now from the poolies event I can by either 6 - 6* Crystal + 4 - 5* Nexus
Or 1 - 6* Crystal + 8 - 5* Nexus
Or any combination in between

Which one should I go for. I want to grow my mystic n skill roster

Skalamenko

6 - 6* + 4 - 5* Nexus

Kinda good option also to get rank up gems for 6*.