I think the choice depends on the answer to 2 questions:
1) how much do you care about autocross?
2) are you prepared to run 100 octane at autocross events?
If the answers are "a lot" and "no", then you really have no choice, get a VF39/43 (whatever you can find cheap) and stay in street prepared.
If the answer to number 1 is no, just get a 20G or green sized turbo and have a great track car. If you are prepared to run 100 octane I think either a 20G or green will actually be competitive in SM, especially at a local level.
nasioc link with charts. For either sized turbo you will probably need injectors/intercooler. If you add 100 octane you will probably need a bigger maf housing as well.
Don't waste your time with an 18G. It is way too small for the 2.5L engine. It will be in the same ballpark as the stock turbo but in a different class.
Or you could just do the sensible thing and ask Mike W for his opinion
