I think it all depends on what you play: clean, distorted; metal; country.
If you play LOUD you will need stiffer speakers and probably want ceramic magnets.
If you play clean and at medium volumes (clubs, small venues) I would highly recommend going with alnico speakers, they are considered to be more musical.
If you are heavy metal or whatever you are probably quite loud. There is always enough bass when you are loud and ceramics give you a tighter bottom. When you are playing at more human levels, the bass can be lacking and that's where alnico comes in.
Use speakers that will handle your output without blowing up, but if you want sustain don't get speakers designed for super high output. for a 2o watt amp 40 watts or 50 watts rated on the speaker will probably give you better sustain than a 90w or 120w speaker, because the higher power rating will mean a stiffer speaker.
I play clean at medium volumes, I like alnico and getting rid of the original stock Eminence speaker was a great decision for me. I am using a ceramic Tone Tubby rated at 40 watts in my 35w Maverick, paired with an extension cab in a mini-stack. The extra speaker is a 12" alnico tone tubby and the combination kicks ***. Sustain all day with beautiful singing distortion. I highly recommend hempcone speakers! They are expensive but worth it and they may require up to 50 hours to be completely broken in, although they lose most of their harshness after 5 or 10 hours.