Is your song a ballad or up-tempo or groove song? Ballads tend to focus more on melody and lyric. A simple piano/vocal or guitar/vocal can be very effective. Up-tempo fun songs often need a bigger production to caption the fun feeling of the song.
Are you pitching to male or female artist? If you are pitching a song that could be recorded by both male and female artists but can only afford 1 singer and 1 version of the song, always choose to hire a male singer. It is harder for a male artist to hear a female singer on the demo and picture himself singing that song. Female artists tend to be more able to hear across genders.
What is your budget? If you would like a full band production but can only afford a guitarist and a vocalist, then make it a great singer and guitarist. Do it simple but do it professional.
Are my own playing and singing skills good enough or do I need to hire someone? Some people have talent to program great tracks that are exciting; or sing and play guitar and are really good at it. If these are not your strengths, then be honest with yourself and get help in these areas. Even a great singer needs to make sure their voice is the right voice for the song. An R&B vocal on a rock song may not be appropriate.
Are the essential elements of the song showcased well when your demo is finished? Are all the lyrics audible? Or is that guitar riff too loud and you can’t hear the words? If the song is built around a bass line, is that bass line punchy? If your song is a fun up-tempo song, is the final demo making you smile? Or is the tempo to slow and taking the fun vibe away? The bottom line is when you get the chance to have your song heard, your demo needs to turn that producer or publisher on.
If your goal is to get a recording artist to cut your song, then your demo is crucial. A publisher or producer may have heard 200 songs the day your song crosses their desk and yours needs to make them glad they listened to song 201 that day!