Yeah, ART pay is lackluster compared to AD/AGR for the same effort and work performed, especially O-4 and above. It all stems from the fact a large portion (BAH is the game changer) of full-time military pay is tax free. In addition, ART new hires take a 4.4% shave from gross pay on the spot in order to fund their FERS annuity portion. 4.4% as of Jan '14 new hires, you read that right. In addition, FEHB is expensive compared to Tricare Prime or standard, and being an ART makes you ineligible for TRS (Tricare Reserve Select aka Tricare Standard for TRs), which is much more competitive than Blue cross Blue Shield family premium or equivalent. So that's an additional effective pay cut you see as an ART versus your AD/AGRs.
Don't get me started on the retirement. 1% pension with 4.4% funding every year vs. 2.5% pension payable immediately and 0% employee funding. Nuff said. Plus in the ART job you have to squeeze your IDT in your normal work week, the AGR just gets paid straight. PLus, if you're doing the ART thing as an ejection seat guy, yeah good luck doing it til 57... Joke. The whole thing is just not even close.
BUT, the ART is more common across the force, and thus gains you access to better locations in the aggregate. Not an inconsequential factor for many. In addition, ART is a much better job these days from which to springboard into an airline job. Always has been a superior gig on that account. Of course, therein lies the irony of having a job be considered great by virtue of being easier to quit...
If you have a high earning spouse, ART is usually a better QOL option for the family based on location alone. AGRs are located in some of the more inhospitable locations for a reason (ask me how I know...). Opportunity costs abound, but working longer for less pay and worse retirement is something one does for reasons of QOL, certainly not as a primary breadwinner if one can help it. It's a paycut no matter how you slice it. Concur with the other posters, at O-4 and beyond it's anywhere between 1.5k-2k net home pay difference after civil service, healthcare and taxable income deductions. That's a lot of coin.