Which one sponser green card?

看你喜欢怎样的工作环境了.
CPA FIRM学到的东西多一些, 辛苦一点;
CORPORATE ACCT 清闲, 但有可能BORING.
不过你说的这个CPA FIRM好象是太小了些, 学到的东西可能很有限.

当然要去大公司，受经济变化的冲击小，比较稳定。去个那么小的公司，万一哪天关门了，身份怎么办？

Is the first offer for staff auditor? since you mentioned it is a cpa firm.
If that is the case, there are two points,
1. As auditor, you will get more exposure to financial/accounting system, while accounting staff usually deal with only one account on the financial statements.
2. Big companies have better reputation, and you probably will get a better background out of some big names for your future career development.