To answer the original question, you absolutely shouldn't buy any ICP-MS (or any other instrument) based only on user recommendations (although these should be a part of your evaluation). You should check out the actual performance of the available systems to see which one is most suited to your application, not just believe what the sales people say.
This means visiting the demo labs of each vendor and taking along some of the toughest samples you might have to run. Run them the way you actually work in your lab, such as trying to replicate the amount of method development time you get. Instrument vendors like to have a week or 2 to "play" with new samples and work up a method that shows their instrument in its best possible light, but this is no use to you if you have to get data back to a client in 2 days. Turn up on the day of the demo and pull some difficult samples out of your bag, and ask them to run them in front of you so you can see all the steps of developing the method. The vendor's response will tell you a lot about the confidence they have in their equipment, and running samples this way will also tell you a lot more about the actual use and performance of the instrument in your lab.
To compare the 2 vendors you mentioned (I've used ICP-MS from both these companies), I would say the main differences are in:
1) The way the collision/reaction cell removes interferences. As you said, the Agilent uses an inert cell gas (He), whereas the Thermo uses a reactive gas (H2 mix). In a clean sample, this won't make much difference, but reactive cell gases cause a lot of problems in high-matrix samples.
2) The matrix tolerance. The Agilent 7700x has a device called HMI (high matrix introduction) that dilutes the sample aerosol. It means you can run samples at 10x higher matrix level than on a non-HMI system. The Thermo also has a high matrix version, but to use it you have to switch off the plasma and change to a different sample and skimmer cone, so you can't run mixed matrix samples in the same run.
On some other points, the Thermo ICP-MS software looks good, but the Agilent MassHunter software is pretty good now as well (they also now use a single table for all data).
Agilent might have been a GC/GCMS (and LC) specialist company 15 years ago, but they have been ICP-MS market leader for years now. For your instrument you should look at the performance, reliability and support of the current products, not the history of the company.
The other ICP-MS instruments available are the PE and the Bruker (used to be Varian, until Varian was bought by Agilent). The PE uses a reaction cell (DRC) so it is not so good at removing interferences in complex samples. The Bruker doesn't have a collision/reaction cell. Evaluate all 4 systems if you want a clear picture of what's available, and if you have the time to go visit the demo labs.