To sharpen your skills, I'd say focus on one off models and spend weeks with them. I didn't really start getting deep into painting until I dedicated 3 months on one model (granted it was massive, you can see it in my gallery).
Painting an entire army to that level, more so when learning, might burn you out. I would say just search the internet for models you adore and learn a different trait or skill on that model (dynamic basing,
NMM,
OSL, etc.)
If it must be an army you want to play, build an allies list that will work with your current army, something that you can work towards getting on the table without shooting for 1500.
Unfortunately I don't believe either of the two you chose work well with that situation. Daemons offer a lot of variety in painting skills (metals, flesh tones, etc.), but Bugs are kinda 3 color chumps and are expected to be done in droves. You're not going to want to do a bizilliion gaunts at above table top; a swarm army is not a good choice for what you're shooting for! Daemons are a bit hordish too.
I hope that helps, I don't think anyone can pick what you like.
EDIT: Also, find a local painting club or competition and then find the winners (or guys with painting style you enjoy) and learn from them. Investing in quality supplies and learning about how even weather can change your painting helps. Things that I didn't invest in initially but have helped me greatly are my wet palette and kolinsky sable brushes. The latter can cost almost a full kit for one brush, but oh how long it lasts and how I love it! Also, Masters brush cleaner. It's at most $10 and can extend the life of any brush almost indefinitely.
I guess research first is the best first step