I'll go ahead and say that while a 1st author publication may be "guaranteed" the timeline can be more open ended so you may not have your manuscript accepted before the end of the year. Also publications, esp those at big name journals take forever.
Next as a med student - esp one that isn't interested in doing hardcore research - having ANY publication is fine (1st, 2nd, etc). Unless you're gunning for a top MD/PhD I would say go for whatever opportunity you feel will open the most doors when application time roles around.
Overall as long as you're intellectually contributing to the project and can speak on your achievements with intelligence and ownership, either option will be fine. IMO Amgen is a pretty well known competitive thing for med school so I'd lean that way but either is fine.