I'm a lover of the G6 filters, the only thing they're missing is the built in heater.
I have also had an Oase filter, simply as I wanted the built in heater convenience.
From experience flow from the Oase Biotherm 600 is a trickle compared to the G6 - simple as that. I tried changing back to the G6 as I just didn't think flow was very strong and using the exact same spraybar etc the G6 blasted the soil away and ruined the scape so had to change things a bit. The Oase pre-filter is awful also, to get it out you need a VERY tall cabinet or remove the filter from the cabinet which is a pain. Those two points alone were enough to put me off the Oase - I've no idea why they are so heavily backed by retailers; money maybe?
1. I've had quite a few of these filters and before I closed some aquariums down had 4 of them running at once. There's now only one in the house which will also be coming to a stop in the not too distant future as changing to an AIO system so everything is contained within the aquarium so no hoses to tackle.
2. The pre-filter cartridges will fit the mechanical section only. I did try one of the finer pre-filters in the mechanical section but it did restrict flow after several months where things had just clogged it up despite keeping them clean so save your money and stick to the normal ones. I've hard water so was possibly due to that.
The chemical cartridge provided is full of carbon but you can take the covers off and providing it's appropriate put what you want in there.
3. I did use Purigen very briefly but seem to favour activated carbon myself, why? - who knows but think it's the cleaning of the Purigen and how easy it is to use a good quality activated carbon such as Rowa Carbon and not all that expensive. *I had previously added Purigen directly to the chemical cartridge which I read people had done but was a very bad idea, the cartridge slots are not fine enough so stick to putting it in bags first!
4. Yes, flow goes from the inside to outside of the pre-filter so things get caught within the inside of it, including lots of shrimp 'safely' I'm unsure is the correct word as the flow is still going into the pre-filter but providing you rinse it with each water change then you should be ok; I've only ever had a handful of dead babies.
There's an art to cleaning the pre-filters and I found the brush most suited to using the handle to tap the cap out of the pre-filter and noting more! Once you've tapped the cap out over a small bucket you will be able to tap the shrimp out that haven't fallen already and found the best way of cleaning them was to just use a hose or shower on the jet type setting and blast it from the outside if this makes sense?
5. No experience of the Eheim 2028 but can compare the Oase as I have above. They say the 'pump performance' is 2460 LPH and actual flow rate of the G6 is 1000 LPH.
One thing I would say as a big negative about the G6 is Fluval advertise you're able to regulate flow with the Aquastop lever without harm to the motor etc which is not true and I've had a couple of the filters unable to reach full flow after running them at reduced flow while things were settling in. Hagen (UK) did replace them for me but I was told the valve regulation is meant for such times as feeding alone which it makes no mention of anywhere on the
dedicated G-series website.
Quote from webpage:
'With its proprietary technology the AquaStop system makes maintenance easy, allowing you to stop the water flow by simply bringing the AquaStop valve lever into an upright position. By lifting the AquaStop release lever, the complete assembly can be conveniently removed without separating the hosing. The AquaStop valve lever can also be used to regulate the water flow with no harm to the motor or its components.'
I'd choose the G6 personally but that's just my opinion.
If you do go down that route and need any spares give me a shout as I've got loads I'll be selling when the other one is shut down and sold on.