Want you to be aware of an addition to your calculus: It is permissible for an employer to have job conditions on a particular position other than pure seniority. In other words, a regional airlines can impose a requirements on hours flown as a first officer before being allowed into the Captain seat. Your future employer may have these requirements imposed by corporate legal etc. If so, you may come back to your airline, have the seniority to be a captain, but NOT have enough flight time at said company to go directly to the left seat. Not a violation of USERRA law to have these restrictions if they would apply to military and non military employees (pilots) alike.
Would I try to go to a regional now? Yes. Gain what ever track you prefer. Once headed military, protected by USERRA where you are currently employed. So want the regional job before headed to OTS and UPT. As Rickair pointed out, would be eligible for any flow through with USERRA protections.