This is my development perspective of these two projects and the current teams that are behind them. I'll try to be as neutral as possible between the two and let you come up with your own conclusion.
With the current iteration of Rage Multiplayer the sync is questionable. It is incredibly noticeable when you do any form of melee combat. Gun accuracy is non-existent and not that great at best. However, they are working towards a new synchronization solution that resolves most of these problems. There is no public evidence of the new sync handling more than 100+ players at a time and being stable.
With the current iteration of alt:V the sync is great. It noticeable from the get go that head shots land, that fist fights work, and evidently better than what GTA: Online has to offer. They are actively working and improving sync with each beta release.
The last RAGE:MP update was back in December of 2018. The developers are working on synchronization issues; but no public update has been release in over 6 months. It is unknown what other features the developers are working on at this time other than synchronization.
At the time of this post; the last update was released today for their development branch. They merge to the master branch quite often and are very transparent about the content they are working on.
It is non-existent; there is no information about what these developers are currently working on other than synchronization. In retrospect there are only two developers actively working on this project and they may not have time to answer everyone's questions.
These developers are very transparent about their road map which can be found in their various Github repositories. You just have to read the .MD files provided in this repository.
Rage:MP is a merger of the old GTANetwork project from 2-3 years ago with a combination of the existing Rage:MP project. This project has been in development for a long time.
Prior to alt:V it used to go under a few different names: FiveNet, and GTA:Orange. They pruned several developers from their team when the team was overly toxic. They've since completely changed their act and have made steady progress since these changes.
Rage:MP has a large community but no where near what FiveM has. alt:V is under that same umbrella of not having a community as large as FiveM. Both of these projects have their very own Discords and the developers actively post in each community.
On a personal note; getting help with development in each of the communities is a vastly different experience. In alt:V you can get your questions answered even on general development. In Rage:MP you'll get sub-par assistance with minor questions and potentially banned for asking arbitrary questions about general development.
Example of incomplete pages: https://wiki.rage.mp/index.php?title=Client-side_CSharp_functions
alt:V takes a little bit of reading to get the initial setup of your resource started. When you download the server files you can also download examples as well; which helps out initially. There are various boiler plates available in their resources section to help you get the general feel of the structure of this client. Structure wise when you're working on a resource you can easily separate your client files and server files per resource. Which is great for organization. I have not experienced the C# side of this client yet; but others have had great experiences with it. There are also Type definitions available if you would like to work in Typescript and compile down to JS.
Initially RAGE servers take up plenty of CPU/RAM usage. If you don't have high-end hardware you mind your servers performance to be sub-optimal. Its hard to say what the general performance of the server is with more than 200+ players at a time.
There is a bit of information on the performance of alt:V. Colshapes take around 9KB of RAM and general CPU usage is very low. At 200 players you can roughly expect around 20% CPU usage, 150MB RAM usage, 3Mbps Outgoing Connection, 1 Mbps Incoming Connection. Which is incredibly low for the amount of players in the server.
While RAGE has been in development for quite some time; it falls flat on updates. They've failed to be transparent with their community about what they are currently working on and what their general road map is. There is no community input on the development path of RAGE. We have no idea what to expect from this team in the future. We've even seen an entire server completely unable to broadcast their server to the RAGE master-list after the owner was banned from their Discord. Getting started with Rage:MP is a generally painless process but depending on which language you choose; you will find the information to be slightly out of date.
Project alt:V may been the new kid on the block; but they have been consistent on their updates and kept the communities best interests in mind. As well as worked constantly towards updating their client as often as possible. Usually a new development update is available every day. They even speak to their community about how different API features should be handled with various opinions being discussed. You can even find one of the developers streaming what they are working on from time to time. From a development perspective it took a little bit for me to get started; but that's because I was completely used to development with RAGE. I can safely say that my experience with alt:V so far has been refreshing.