According to the guides, Era 100 or 300’s should be at least 20cm from the side wall so you’re fine on that consideration. 300 has up-firing speakers so you’ll prob have better Atmos sound. Budget is your other consideration. 
If you have access to an iPhone, run Trueplay and see if that makes much difference with/without: it may help with the spatial differences in placements. 

Oh didn’t know that it had to be at least 20cm from the wall

that I can do

so should probably go with the 300 then

 If you want the benefits of Dolby Atmos get the 300s.  Trueplay will take care of most of that asymmetrical layout.  My surround setup also has one surround closer to me.  I’m using 300s with great success.  One reason I think this works better with the 300s is the front firing speaker is disabled when they are used as surrounds.  
 
 There is one other thing that influenced my decision to purchase 300s rather than 100s.  I’m a big fan of orchestral music.  Orchestral music in Dolby Atmos is awesome.  The stage is much bigger than it is when played through stereo.  So if you are not into Orchestral music or you don’t use either Apple Music / Amazon Music for access to Atmos or budget is a concern then the 100s will work fine as surrounds.  

If truplay takes care of most of the asymmetrical problems the I think I should go with the 300

its not like moneys is not a problem but I don’t mind paying for the 300 if the give me a much better result

 Once you are set up and if you have either Amazon Music or Apple Music listen to the Dolby Atmos album John Williams: A Gathering of Friends.  It has awesome sound.  The sound stage is massive.  Even if your not a fan of classical music that album will at least show the benefits of Dolby Atmos surround processing.

Will do

i owned the Era 100 and 300, to me 300 more clearity of voical and background musics.

comparision of both of them.

100 more bassy and punchy with lesser details.

300 more clearly and details, just bring you to another world.