Hi
Both are equally good AVRs.....output is more or less the same, similar features....Onkyo is THX certified....(blends well when you use THX certified speakers- these are expensive). Both have ATMOS and DTS X which are relatively new features since the last 18 months and is still developing (This too required specific designed speakers, placement...will increase the cost...)
Overall feedback is that Onkyo has no proper Service in India and some have faced challenges.....This is where Marantz shines out as a winner according to me and what I heard around.
I assume you are not happy with the performance of the Denon 1200w (Manufactured by D & M Holdings...also makers of Marantz) and maybe looking to upgrade....
Based on your signature information, you are using Q Acoustics 3000 HT speakers...these are rated at 75 watts @ 6ohms (Fronts + Surrounds) and Center is 100watts @ 6 ohms...these are really nice decent speakers for movies and music....read many positive reviews....all they need is a good matching AVR to exploit their potential....
The AVR models you have shortlisted will output 140watts minimum at 6 ohms...(overkill for these speakers..unless you are planning to upgrade your speakers too).
You should look at AVRs which can deliver 50watts - 100watts @ 6 ohms load.....I believe a Marantz would take care of this and rather at looking at SR series...why not the NR series? The NR series provides continuous 70watts rms @ 6 ohms which makes it perfect for your speakers.....I have used various models and is a very happy satisfied user.
If I were you, I would immediately sell the 1200 ...buy a NR series Marantz which is good enough for the Q Acoustics .....and also the NR series slim body looks elegant with stellar performance...
Also remember the AVR market keeps competing with each other...some features, certification, gimmicks will always come and go....models will change.....6 months - 12 months....
You can look at buying a new NR series or a pre owned one....I believe there are some NR ads in Hifiads.com
If you are not into Dolby Atmos / DTS X for the moment, I would advise to look at little older models which may be a bit cheaper and have the basic features (Wifi / Bluetooth / 4k upscaling / Audyssey / etc). You should aim for the AVR features which you will use...
You can use the NR series for a while and when something else better comes possible next year and is well established, you can then consider the extra budget required and invest accordingly to that upgrade....
End of the day, you need to evaluate the pros and cons and make an informed decision.
