Keep in mind, being first author is inherently more time-consuming. I spent many long nights during second year writing, citing, revising, and analyzing data. You can get away with being second author with much less work. With respect to "shoot for first" you generally do have the ability to make it known to your PI that you are interested in the challenge. If the lab is big, you will most likely get tacked on to other papers while your also authoring the main paper (albeit, it will be somewhere in the middle line-up of authors).
My recommendation is make it known you are up for the challenge, make your presence aware in the lab and get that first author publication and while you're at it, get tacked on to a couple more papers as you exchange your draft with other lab members, and help them iron out their paper. 1st author + multiple papers with mid-author placement >> 2nd author.
Note: this is possible because being 1st author means you are going to be heavily involved and thus have more opportunity to work with the lab members; vs. lower-rank author med-students who come in a couple times a week to run a blot.