The SRX-SYS-JE SKUs only provides a perpetual license for Application security which is also included in the flex subscriptions (both A1, A2, A3, P1, P2 and P3) - so if you need flex subscriptions anyway, then just go with SYS-JB hardware SKUs.
secondly the SYS-JE will probably be phased out soon... so in general always look at SYS-JB + flex licenses on all new installations.