What are you going to be using it for OP? If it’s for sitting around outside, down works better for retaining heat when static and just feels nicer than synthetic.
As other have said synthetic works better in changeable and crappy weather, particularly if a active / semi active.
The hydrophobic downs and improved dwr’s on down jackets still wet out fairly easily. A few brands do downs with proper hardshell outers so far more weather proof, but they cost a fortune, are heavy and too warm when moving unless really cold weather.
If your looking at synthetic i think it’s pretty hard to beat primaloft fills, especially the gold prima loft (although silver isn’t a mile off). Hagloffs own synthetic fill is pretty good too. I’d look at something with 100gsm fill upwards. Things like the montane prism are c 50gsm so makes them great versatile jackets when moving, but isnt warm enough when static for any legnth of time if it’s cold
Having owned many synthetic jackets of different weights, brands and intended uses, my favourite by far for general outdoor mooching / camping / sitting around / semi active is my mountain equipment Fitzroy. Good balance of warmth, weight, pacability and the dry-lite outer works well in the rain
Outdoor kit prices seem to of got silly in recent years, although I suspect they’ll be some decent retailer discounts about if you shop around. Other option is try outdoor gear exchange group on Facebook or e bay for second hand. Most outdoorists look after their kit well and have jacket habits so some decent prices to be had when people having clear outs
