Based on my research I think that Anonaddy and SimpleLogin are among the best e-mail aliasing services, they share most things in common, but have some differences as well.
Anonaddy:
+ Better Free Plan: Anonaddy’s free plan is a real plan, it offers much more than SimpleLogin’s free plan which is limited to the point of essentially being a free trial.
+ ‘Light’ Plan: Anonaddy’s light plan is the sweet spot for most cost conscious users/individuals. It costs 1/3 what SimpleLogin’s premium plan costs–SL’s plan offers a bit more but the Anonaddy Light plan should be sufficient for most people.
- Premium plan is slighty more expensive than SL I think (but most users can stick to the Light plan which is substantively cheaper)
- Smaller project / single developer: This isn’t necessarily a negative, but it does make me slightly less confident that Anonaddy will exist (and be actively developed and sustainable) in 5 years, 10 years, 20 years when compared with Simple Login / Proton.
Simple Login
+ Owned/backed/developed by Proton. This gives me somewhat more confidence that the service will be around long term, and actively developed (albeit slowly probably, but that is ok).
+ Can use it for SSO on some sites (like this one) if that is something you use/care about
+ & - Proton is a company with a lot of different services, that can be a positive if there are useful integrations or cost efficiencies, that can be bad if the companies focus strays from SL to other services or if development resources are spread too thin
- No equivalent to the light plan offered by AnonAddy.
Possible + for Protonmail users, having your aliasing service and e-mail on the same infrastructure may offer some security and privacy advantages.