I have not used Sentinel controllers, however I do use the CAP PPM-3, and an older version of that cap controller you listed. Sentinel controllers are supposed to be quite good, however priced at a premium. For premium controllers I would consider buying Titans.
My experience with the CAP controllers is you get what you pay for. I had intentions of buying Titan or Sentinels myself, however my store was selling CAP stuff 50% off, because they were discontinuing them. I guess CAP was giving them a hard time and they were getting alot of returns. I figured at half price, even if they broke, I could get some good use out of them during the warranty period for a good value.
My controller seems to work well, but as you said the CAP controller doesn't manage CO2 on its own, you need the Monitor/Regulator. My CAP PPM-3 was glitchy, and it would lock up every now and then eventually at least once a day, either locking CO2 on, or off until I reset it. I guess this is a common problem. I fixed this issue by putting a timer on it that would turn off my controller for 1 minute every 3 hours. I've been going 3 weeks straight with no issue, so I guess its workable.
In regards to the actual controller, it seems to work fine so far, if you can find it at a good deal, might be ok. Its nothing special, definitely cheaper contruction, but I guess thats why its cheaper priced.
I guess my reccomendation is you will get what you pay for in regards to these brands I guess. I've made my CAP stuff work, but wouldn't have bought them in the first place had they not been discounted so heavily. Considering you have to buy the controller and co2 reg seperate, it might be cheaper to get an all-in-one, or at least comparable in price for better quality.
For the PPM-3, you might be able to find it cheaper if you get a different brand. The ppm-3 is a rebranded/repackaged Telaire product(i think originally made by GE or something).
Another thing for the PPM-3 if you get it, it only supports .5 amps, so you can't plug anything but a solenoid into it without a relay. If you pop your controller with something big, (i know people have done it with pumps on a water cooled system) it technically voids the warranty.
https://www.google.com/#sclient=psy-ab&hl=en&tbm=shop&source=hp&q=telaire+ventostat&pbx=1&oq=telaire+ventostat&aq=f&aqi=&aql=1&gs_sm=e&gs_upl=11110l12589l2l12902l11l10l0l0l0l0l367l1921l0.4.0.4l8l0&bav=on.2,or.r_gc.r_pw.r_cp.,cf.osb&fp=a961623ff45346f0&biw=1366&bih=644
