Sofas are tricky for sure. If you’re sure you want to have one (keep in mind that if your space is small that you may want to go with a couple of comfy singles or perhaps loveseats)…
Probably the first thing to do is consider your budget & how long you plan to have your sofa/stay in your place. Be realistic. Often sofas don’t travel well to other spaces – I have found that each room calls for something different. So perhaps a cheaper yet comfortable sofa with an eclectic mix of pillows would work…unless you can and want to go down the route of a classic design which has already stood the test of time – think chesterfield, or scandinavian mid-century modern for example.
Then think about function – how you will use it. Do you often curl up and read a book or watch TV? Is it for entertaining guests on; do you need it to multitask (like a sofa bed for example)? Is it just for you or are there often more people using it at the same time etc.
Then think about size. An interior designer once told me to make a template out of newspaper and put it on the floor and up the wall (height can also make a difference) so that I could guage exactly how big it would look like in my space. (They always look smaller in the showroom!). Size needs to suit function and fit into your space. You’d like to have about a meter to walk around the whole thing, if possible though you could get away with 60cm (at the absolute smallest) and if you live in an apartment then consider if it will actually be able to get up the stairs (I had this problem once – what a nightmare!).
Finally, once you have decided all that, form. From colour to shape/design, this is the hardest part as you say BUT since you have decided on everything else above then your choices should now be somewhat narrower than when you started. :-) I love mid-century modern furniture but generally those sofas are not the most comfortable to curl up in. Feather and down cushions are super comfy but pricey (though they do stand the test of time). Foam is cheaper but often hard – I have yet to meet a foam sofa that I could really relax in.
I hope I’ve helped rather than make it harder, haha! I have a small space now and in the end have gone with bench seating with storage drawers underneath plus a couple of single seats rather than more sofas. The benches are good when guests visit but often having sofas isn’t essential. Sometimes you do have to compromise to get the “best fit” for your space, budget and style.
Good luck and let us know how you go! :-)