I use both and I'll give you my honest opinion:
As it stands today Active Sky in addition to the weather engine has a lot of useful things that OpusFSX does not currently have, such as Maps, Met/Reps, Weather finder, etc., all in a single application
which can run on a Network client. No need to give a review on this software since most users (and there are many) know its pros and cons.
OpusFSX has a weather generating engine with regular real weather data from NOOA. I am very impressed with the quality of the output the like of which I have never experienced
on any of the sim versions, so in this respect it is tops for me. There is one thing I don't like, and it is not having any weather effects until they are activated by creating one or more camera views
and enabling DHM; would expect effects to emanate from the weather engine automatically. What this means for me is that at the moment I cannot use the OpusFSX just for the weather, without
messing around with camera views, which I feel should be a separate module. My set up comprises one server and one client, and so far the weather engine has not had any noticeable effect on
my FPs, also when weather changes the transition is super smooth something to be very grateful for, but having been a devotee of "FSX only" on the Server PC I feel uneasy but that's just me
breaking a Taboo.
Understand these are early days for the Opus Team and that they have many changes on the pipeline, including a version that will run on a client. So although not happy about the weather effects being separate from engine I intend to continue using it and will wait and see what the future brings from this team. As regards Cameras & Live Views, you have probably gathered I don't care much about them,
but if the effects and quality are as good as the weather generation then I'm sure those of you who are interested in this area won't have any reason to complain.
Also own a registered version of REX. Did test the weather engine a few times but found ASE to be much better and since only use for the textures.