A couple of thoughts since I'm in the same situation. I have the same size pvc enclosure with just an RHP (88 watt). Currently it's empty while my baby BCI is in quarantine; plus I might wait until he puts on size before I move him into the larger enclosure. Regardless, if your going to go with just one heat source, I would hands down go for the RHP. Not only can it create a hot spot but it will create the correct ambients if you have the right size RHP and you spend enough time dialing in the temperatures. Flexwatt will only control the surface temps so you could 'potentially' run into ambient temp as well as cool side surface issues.
I'm curious and a little bit worried about your heat pad overheating the hot spot. Are you using a Thermostat?? If so, I'm not sure how your hot spot is getting any hotter then what you've figured based on your Thermostat setting. For instance, my Tstat in my tub is set to 94 degrees to achieve a surface temp of 88.