I write in Sibelius but I do not like the its sounds. I like Reaper's sounds but its notation editor is not practical. Thus, I export a midi file from sibelius which I import in reaper. However, I find myself repeating too often this process since I make little changes all the time and I need to know how they sound. Is there a more practical way to write and playback music in western notation with a PC?
What you're describing is a pretty typical workflow. Sibelius is a notation program and isn't that concerned with great synthesis. Reaper is a DAW and isn't that concerned about being an awesome notation editor. You can import better sound libraries into your notation program, but then you'll just find something else that's lacking, such as fader automation or equalization.
I think the best thing to do is embrace the imperfect playback of the notation program. Treat it as a tool that's only good for making sure that the music works and you didn't make any silly mistakes. Get used to hearing the playback in Sibelius as an approximation, and train your brain to extrapolate what it will sound like once polished. Do all of your notation first, and once you're sure it's what you want, then take it to the DAW.
I'll echo alephzero's comment: the right way to compose classical music on a PC is to shut the PC down, put a blank sheet on top, and write on that with a pencil. I don't say this out of old-time sentimentality – in fact I do practically everything but music on a computer – or because all the great composers did it this way†, but because from my personal experience, digital helpers are detrimental to the quality of the result: it's far too easy to end up with poorly thought-through lines and overly repetitive structure.
Pencil and paper force you to actually reconsider every detail whenever there's any sort of repetition. Also, they save you from the temptation to ever heed the questionable feedback that a digitally synthesized rendition of your work provides.
So if you do use a notation program, I'd advise you to never use its “play” feature (nor copy/paste). This way you won't need to worry about its sounds, and you won't have you work sabotaged by misleading impressions. To test out stuff, use a guitar/piano/whatever instrument you prefer, and most importantly your imagination. Of course this requires experience, but you'll need that anyway to write good classical-style music.
As a compromise, you can use the play feature but with not the actual intended instrument sounds but also e.g. piano. That won't sound good obviously, but it may give the clues you need and leave more room for “hearing what it would really sound like in your head”.
All that said... this kind of task can be automated. Not sure about Sibelius, but MuseScore and certainly Lilypond can be scripted to always automatically export the MIDI, and Reaper can be scripted too. Should be possible to reload MIDI files with a single keystroke. It's going to be a bit fiddly, but there's a good amount of documentation available.
An even nicer workflow would be possible with an offline-rendering synthesizer: you could set up e.g. a Bash script to always re-render the MIDI file to a wav file, then whenever you need audio feedback just play that file in a standard media player.
†Though they did. (Well, not on a PC actually...)
Is there a more practical way to write and playback music in western notation with a PC?
I am using "Musescore " more than a year now.you can notate and have lot variety of sound for play back. You can even export in .Wav format and import in Garrageband and use for recording as well. Please try once if this fulfill your requirement.