Delta has undergone a significant seniority list expansion already. This doesn't really matter if you're younger than average or have only NB career aspirations. AA is comparatively the largest and while it may be the airline constrained the most with organic growth potential, it also has the highest average age. Thus, a 40yr old hired today at AA will retire with significantly more seniority than at delta, and UA would be somewhere in the middle.

What all this means is, if you know where you want to live and get hired by the airline with a domicile there, the seniority impact is irrelevant because commuting sucks.

To pinpoint your question further though, the best time to get hired is right before organic growth happens. That gives you the best odds of furlough protection, off reserve, upgrade to higher paying aircraft, etc.