Depends on what APIs you need, and what your tolerance is for futzing around with things. If you need the larger API surface of the .NET Framework and you don't have time to be on the bleeding edge, go with Mono. Xamarin Studio has good support for it, and even if you're using some other editor (VSCode, Spacemacs, etc.) everything still generally works with the tooling for Mono.

There are basically two components that need polishing for .NET Core to be a reasonable choice - the implementation of NetStandard 2.0 for .NET Core (so you get all the old APIs back), and for the tooling to mature (transition from project.json back to fsproj). These efforts are coming along nicely [1][2], but still aren't ready for prime time.

Here's a roadmap that Enrico Sada has laid out to get the F# .NET Core situation to maturity: https://gist.github.com/enricosada/6f0a0424ca65b9d08e5a84f79...

And by the way, don't worry too much about switching over from Mono to .NET Core. With NetStandard 2.0 the API changes should be minimal. Similarly, once the cleaned up proj file format is out of preview the change in tooling from Mono to .NET Core should be minimal (or even non-existent).

[1] https://slack-files.com/T04BJKUMU-F2ST232AJ-6e68200a3d - Enrico Sada's work on getting dotnet cli to work with proj system for .NET Core [2] https://twitter.com/sforkmann/status/790488306991661056?s=09 - Steffen Forkmann getting Paket to work with new proj system

My plan is to target .NET Core. I like the idea of being able to run on cheap Linux instances. Although Xamarin is part of Microsoft, I'm concerned Mono might end up being the red-headed stepchild, with Core getting better company backing. The only thing that sucks is that currently some of the larger cool projects (like Suave and Akka.net aren't ready for Core yet (I think)). So eventually: sweet, short-term might be a couple more headaches.

I agree! In the new scheme of things I'm not sure how Mono fits into the .NET landscape. I was eagerly waiting to get a answer in Changelog's latest episode[0] but sadly they didn't cover this at all.

It's even more confusing that there is absolutely no mention of .NET Core on the language's Github page - http://fsharp.github.io/

[0] - https://changelog.com/podcast/224

If you are planning on launching in matter of couple of months, mono. If you are thinking 6 months and beyond, .net Core.

Also, .net Core does not have feature parity with .net4.6/mono, so you may want to know the gaps depending on the type of application being built.