It's good to play some ranked I think. The matches would matter more to the player, you don't surrender as easily and everyone goes a bit harder and the picks aren't just what people feel like even if it's five AD squishies.
So I assume it's the better tactics to improve. And unless you're well below 50% winrate it won't hurt your placement either.
On ranked you will be facing opponents who are placed about as highly as yourself. In normals you get a lot of mixed stuff with people from bronze to diamond all on one match; maybe the normal MMR is just not as exact or premades of multiple guys affect the matchmaking heavily.
I'd still go for normals, though. For example, many people including myself tend to play epecially bad on their first daily match. Or if you've lost a few matches of ranked in a row, it's often better to go for a normal and calm down instead of playing rankeds on full tilt. Of course, if you are trying new tactics, builds and champs you should practice them on normals first or your ranked mates will have every right to be mad at you if you're making it harder for them.
Normals are a good choice to prepare for ranked play at any rate: getting runes pages ready or if someone can't play a role at all, they ought to learn a few champions to play on that role decently on normals, for their own best, (it wouldn't do you good if you lost every single time you were forced to play a specific role; not only would this stop your progression but also annoy your teams).
And these are two good tips I've discovered, at least they seem to work for me:
- Draft Pick normals are more serious than Blind Pick. Less trolls, a proper ban phase, no screaming for roles and the average draft player is simply more skilled than the average blind player, I daresay.
- During night time or before midday (depending on time zones I guess), you can play much better in such elos as Silver. There'll naturally be less raging, trolling and flaming with most kids being at school, the average player's age will simply be raised a bit and I am positive you can feel the difference.
