I hate to be 'that guy' but I would choose whichever one you personally like more. Sure, there is a lot of spider stuff out there but it's because it is a dramatic mutation. Have you seen an adult single gene fire? Not impressive to me. They look cool in combos as babies but I really think that in a few seasons when there are more adults around people will realize it's not that cool of a gene. It does some cool pattern stuff though if you get the right one or do the right combo.
In the end, you're only going to have fun with this if you're producing animals that YOU like... so don't choose because people are saying fires are worth more money right now. They are all the same species - don't base your choice on what OTHER people are willing to pay more money for. You may very well end up keeping babies forever if they can't sell or dropping the price to where the spider combos and fire combos are the same. Base your choice on what YOU want to buy for YOUR collection. Breeding should be secondary IMHO.