If the Mojo Spider is healthy, I'd say definitely go for it. But make sure it's a female because that's a steal of a price. I agree with you that if the price is the same, I'd go for the multi-gene animal versus the single gene. Spider BEL will look the same as regular BEL's except they may have a head hobble. If you're dead set on trying to see if the BEL's carry Spider, get a backlight and you should see the pattern very clearly.

As far as percentages go for creating a BEL, it's the same as just a Mojo and Lesser pairing, except half of them should carry the Spider gene.