Tough call. The best answer would be both and get the regional seniority number first, but at your age if you're going to do the mil you need to do it now while you have an age waiver available.
The pure civilian route would have a *decent* chance of getting you a legacy seniority number sooner. But it's a less certain path, might end up stuck at a regional like so many others for whatever reason. That may be less likely than in the past given the looming retirements, but I wouldn't assume it can't happen...
Mil would probably take a bit longer but would be almost a sure bet with a predictable timeline if you get winged. You would probably want to chase the regionals to build time and check the 121 box once done with mil training/seasoning.
So it's your call, chose the balance between aggressive pursuit of seniority or the safer path to success. Also should consider how bad you want to do mil flying, if you're really into it that would be worth a delay of a couple years IMO. It shouldn't be much of a delay, in the next five years with mil and 121 regional time you should be a hot commodity.