We use the 3M Clean-Trace unit and it has served us very well. The company is very knowledgeable, they support their product and the software is user friendly. I believe they have an updated unit in the works too. We previously used the Charm unit and it was good too but the one thing we didn't care for was the size of the swabs, they took up a lot of room in storage as well as the QA Tech's morning start-up kit.
Once you have narrowed down the brands you like then the company should be willing to let you test one for a few days. On the contract side of things we have found that loaning/leasing from the company has worked best for us rather than actually purchasing the unit.
As stated above, I agree that ATP should just be an additional tool to ensure that the sanitation procedures are being effectively followed each day. But a good tool it is because you get the results instantly. The other plus is that the data can easily be uploaded and analyzed per the company's software. We have found the trended data very useful. One example is that we discovered one piece of equipment had higher NTU's than the ones around it. When we looked into it we realized that we has some sanitary design issues with the equipment. Once the equipment was better designed, the results were great and have continued to be so.
Good luck! Look at it this way - ATP testing will be additional support that you are doing a great job at sanitation.