No reasonbly speaker is capable of equalling the performance of a subwoofer, so for action movies a sub is essential. Even my floorstanders with 3 x 8" drivers, and 36hz F3 do not come close to the subwoofer, and a good 12" ported sub will utterly waste them.
Plus the power demands on a speaker set to "full" is far greater. So you may save money by not getting a subwoofer, but you need more power, or the power you get has higher distortion, degraded sound quality and you get less dynamic range as a result.
Much of the material I watch, or the media sources (nowtv, freeview) don't have 5.1 available
AVR's can expand stereo, and even mono to 7.1 and they do a fanstic job. For example my av pre is a Lexicon MC-8, and even with mono it sounds great. Watching Akira Kurosawa Seven Samurai, battle sounds that are from behind on screen come from behind on the speakers, as the arrows fly and they come from behind you. Amazing stuff. Same for Pro-Logic, it can expand to 7.1 (discrete stereo sides and rears) And it works for stereo concerts, older stereo TV shows etc. Or you could mix mono to stereo, or just leave mono to mono, or downmix 5.1 to stereo.
Hassle of having to install additional speakers, no clearance behind prinicipal listening position and would not want a speaker 1m from my ear (I think surround sound speakers may also compromise sound quality significantly, they have to achieve form over function so that people will accept them in the standard living room, also sceptical that equalization capabilities of A/V amps are genuinely able to accomodate for room)
Not really much more of a hassle, just speaker wire across the room and it's done once. Also if your rear/side speakers are close to you, I would recommend bipole speakers. I'm about 1M from side/rears and I use bipoles..
Since surround speakers have more drivers per speaker, they typically cost more than the similar range front speakers. For example if you had Kef Q15's for fronts, then the dipole rears probably cost £400. So I would recommend second hand surround speakers. There is some form over function with surrounds, however since the sound is mostly effects not speech, then "standmount" style is sufficient (treble x 2, midrange/woofer x2) as I doubt anyone could get away with using 50kg, 1.5' wide & deep, chest height floorstanders for side and rears.
woudl invest in an oled/better quality tv before 5.1 too.
Sound is 50% of the home theatre. So why would you compromise sound?
For a smaller room, I'd get a midrange AVR, say £350 Yamaha, with Q Acoustics 2000/3000 series, bipole rears, and 10" or 12" sealed box sub.
I don't think you need to go over a 6" woofer for that size room.
I have similar sized room. I use a 13.5" 1000W sub. Sound fab. 6" is little more than a lower midrange woofer. I'd get something like the XLS200 as a minimum.
I have never wanted for a subwoofer with my floorstanding front speakers as my viewing/listening room has always been small.
For a home cinema, you should get a subwoofer. The only reason I wouldn't get a subwoofer if this was for music, pure 2 channel system only with floorstanders, or you just aren't into action movies. But if you have anything smaller than standmounts for a movie system a sub is essential.
You can get away with not having a sub in certain circumstances. For example for my PC system I'm using a AVR, with large standmounts, no sub. I'm close to it so a sub will be too much. Or in the other room I floorstanders in hifi, so sub isn't needed