You can get either an ASN or a BSN to be an RN...2 year vs 4 year programs, respectively. But they are both RNs, and you take the same test at the end.
Where I live, there is no pay difference between the two.
Some individuals who already have a BA/BS in something else, opt for the 2 year program.
If you think you might ever want to get into management, or go on to advanced practice nursing (master's degree level), it would be wise to get the BSN. Also, more employers are now requiring, or would prefer, the BSN. One of the schools near me is actually donig away with their ASN program next year.
In my post-work haze of fatigue, I see that you want to go back for your masters...so I would strongly recommend the BSN!