Both subs are excellent performers and each one does some things better than the other so it does become a matter of what attributes are more important. Having had the opportunity to critically listen to both subs in two shootouts over several hours with a half dozen friends I'd make the following observations.
JL113 outperforms the Ultra-13 in articulation, musical presentation, WAF, mid to upper bass output, headroom and bandwidth, attack and decay of bass transients, quality and reliability of amp, lower noise and distortion, one shot equalization, no port chuffing. Contrary to some reports, output as measured using the CEA2010 standard favours the JL113 except when the Ultra-13 is in maximum output mode tuned at 20hz where both subs have the same distortion weighted output.
Ultra-13 bests the JL113 in visceral impact, reproduction of organ music, low bass output and extension, variable tuning, significantly lower price .
Overall the consensus favourite at the shootouts was the JL113 based on sonic quality and overall performance in music and HT but that quality comes at a big difference in price. With all due respect to other people's ranking, the Ultra-13 is a fine sub but its not the equal of the JL113.