Had a G6 and a G3 for years. Never had a problem with either. Great filters. Very reliable, very easy to maintain. The pre-filter is likely to clog on a heavily planted tank quickly. Mine needs to be changed weekly, which takes 2 minutes to do. I have multiple cartridges and just replace when necessary and clean the old one when I get around to it. Flow stays high, water crystal clear.
For full maintenance, it's easier than the XP, or Eheim 2076 that I had previously because the gasket seats much more securely and the locks are much more positive and sturdy than other canisters.
The G3 Pre-filter is only changed once every 4-6 months. It's a less heavily planted tank, but still has a heavy fish load. No problems ever.
The only challenge with the G6 for your application is throughput. I think it would be borderline fir a 120. Having said that, a single canister of any kind is borderline for a heavily loaded 120. My G6 has more flow than the equivalent Rena and Eheim because flow is always at max since the pre-filter is so easy to maintain. In the other, traditional designs, the filters clog gradually over time, So flow is below max most of the time.
FX5/6 would be good, but they are big, heavy filters to lift out of a cabinet while full of water. Not so easy to maintain. The Gs are fast and easy compared to all equivalent models.
You might want to consider a sump for large capacity and ease of maintenance.
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Eheim Pimp Club member #414, 2076 Pro 3e