Should I run 2 regular circuits or a subpanel to my septic site?
Jay, you have a couple of requirements here.  First is to supply 240VAC to your pump. Since 140 feet is a long run and the pump is rated at 15 amps, it would be wise to oversize the wire gauge to 10/3 UF or DB twisted AWG. This will avoid excessive voltage drop, thus protecting your pump. You must still have it fused at the main box at the recommended amp size breaker. Do not put a higher rated circuit breaker than the pump requires. This would allow pump to overheat before reaching the amps necessary to trip the breaker. The same theory applies to the 120VAC GFIC outlet. Oversize the conductors, but do not overfuse at the panel.

A weather tight subpanel would be a good way to go,  if you can absorb the extra cost. It will allow you to have breakers closer to your pump and outlets. This really would only be for the convenience to not having to walk a distance to reset them if you are working in the pump area. As Niall  already said, you will only save one run of 12/2 or 10/2 and one breaker location in the main with a subpanel, so there is really no financial advantage.

Be sure to pull an extra fish line and leave it in the conduit for future use or maintenance, and don't forget your caution ribbon just below ground grade when you backfill your trench.  Good Luck.