Pros
- C3 Runtime.
- Working Built-in Mobile addons.
- Construct 3 Sprite-sheeting. Improves performance, but (See "Cons").
- Cloud Exporters. Both Mobile and NWJS.
- Quick bug report support.
- Well supported and future upgrades.
- Feature requests.
- New Construct 3 built-in exclusive addons.
- Construct 3 Editor adjusts to computer resources unlike Construct 2.
- Construct 3 Editor increases performance if available.
- Construct 3 Editor responds well and scales to GPU High Performance boost.
- Construct 3 doesn't load all icons at the same time. Hence, supports large projects.
- Construct 3 Editor has great support for large projects.
- New coming 3rd party addons that only supports Construct 3.
- Construct 3 assures your project's future especially for large long-term projects.
- Great themes, Dark Theme.
- Responsive Editor.
- Editor Multi-Screen support.
- Somehow filtered non-supported or well made addons. Because, not ported to C3Runtime.
- Construct 3 SDK is improving and professionally documented as the software progresses.
- Remove dependency on engine bound addons. Which is good for future-proofing large projects. Which is good in its own way. (See Cons)
- Construct 3 SDK support Editor extensions.
- Construct 3 Editor plans to support the creation of addons through events in the future.
- Cleaned C2Runtime code.
- Removed Canvas2D support, which bloats the runtime.
- Removed JQuery dependency.
- Future transition to Web Workers. For performance and efficiency.
- All WebGL and transitioning to WebGL2.
- The use of Web Assembly for CPU extensive processing features.
- Live previews.
- Lots of C2 Editor bugs like variable corruption, ribbon, etc are fixed in C3.
- Subscription means additional assurance for continuous support.
- Subscription means assured continuous support.
Cons
- No MadSpy Steam addon.
- No Spriter addon for Construct 3 Runtime, yet.
- Construct 3 Sprite-sheeting getting worse every update, but (See "Pros").
- Some addons can't port to C3Runtime. Mostly engine-bound.
- Construct 3 SDK can't extend to engine features. Which is bad for small games that rely on experimentation. (See Pros)
- Construct 3 is slower in weaker devices because of Browser overhead.
- Construct 3 SDK is limited to available documentation.
It's not a clear comparison because this is on the spot. But, I think this gives an idea.
I think the comparison is clearer once you have read Construct's own comparison.
Link : construct.net/en/make-games/games-editor
Just in case you haven't read it yet.
Conclusion
Construct 3 is the future with its new features, updates, upgrades and support available in the present and in the future especially with the C3Runtime, Construct Team's own Independently made Editor, New Addons, New coming 3rd party addons, Support, Feature Request and Runtime cleanup and factoring. This is beneficial for the long-term including long-term projects and learning it the soonest will ensure you that you will be familiar to the engine when you need it the most.
The problem is Construct 3 is unstable, because it is constantly upgrading unlike Construct 2 and things might become great and bad depending on the situation like the Construct 3 Sprite-sheet algorithm which became bad or okay for some and worse for others. Also, old addons can't be used on Construct 3 if they aren't maintained and migrated to Construct 3 especially the engine bound ones but that is a good news in itself so you can avoid old, non-supported and experimental engine-bound addons that aren't upgraded to Construct 3 and C3Runtime.
If it's not maintained, it might not be a good idea to add in your project at all. Especially if new 3rd party addons are made or the Construct Team makes a built-in feature for those old but popular addon like the newly released ones for the C3 Runtime.
I hope that gives you and idea.
Have a nice day and happy game development!