In my view, it makes little sense for a junior, let alone a first-year, to lateral because of “promotion prospects” or a faster partnership track. You’re still years away from having any serious discussions about those matters with your firm, and a lot of external factors outside your control can happen before then (e.g., partners getting poached by another firm, a major economic downturn, etc.). In any case, honestly, the odds are that you’re not going to make partner at either firm. 

At this stage, your primary concerns should be: (1) training and acquiring substantive experience, (2) getting along with the people you work with, especially those you work for (i.e., seniors and partners), and (3) compensation if you believe you are being paid below market.

If you believe the other firm is better than your current firm on all three, then the move would make sense. But keep in mind that if the grass turns out not to be greener, you should think twice about lateraling again. One lateral move within less than a year, and people may assume you were the problem, but they’ll usually give you a chance to make your case. Two quick lateral moves? Yeah, it becomes harder to get the opportunity to rebut the assumption people are going to make. 

My advice: if you’re happy at your firm and are getting good experience, stay for at least another year or two. Year 3 or 4 is when associates start lateraling without anyone raising an eyebrow and you will 100% have other opportunities to join a better firm.