Separate Genres

Agents are not going to love you for trying to write in 2 very distinct genres.
Query for the mystery - there are more publishers for mysteries than SF

Remember you, if you get an agent, become like a company i.e. Joe Bloggs plc. Your agent will be selling your product, he/she will need to know what market you intend to break into.

Establishing a name in two different genres with two different publishers is more difficult.
It is struggle enough to get established in one genre.
You are not going to bring much of your readership from one genre to the second so you are starting out as a new author again when your first book in the second genre appears.
IF you can sell well in both genres then it can be more profitable, but remember two different publishers mean two sets of deadlines.

Query your strongest work. Or the one you'd prefer to write in if you had to choose. There are agents out there who do both, and you can start with those.

The downside to writing in two different genres is that you're basically starting over as a new author in both and the readership rarely crosses over. So you have to work twice as hard to build an audience. If you have a blog that focus on the sci fi, the mystery folks won't read it and vice versa. Any buzz you build for one genre won't translate to the other. Conferences and appearances may overlap and you'd have to choose which to focus on.

However, plenty of authors do write in different genres, using different names, so if this is what you love, there's nothing wrong with going for it. Just be aware of the additional challenges you'll face.

And you can always write sci fi mysteries Genre crossovers are growing more popular every day.

I have a friend who got "The Call" from an agent and one of the first questions she asked was "what else are you working on?"

When he told her, she was really disappointed because his other projects were in different genres.

She told him he has to decide which shelf in the bookstore he wants to see his books on.

That's providing every single book you wrote is marketable and sells. They may all be publishable, but most people never publish the first few books they write, because they aren't professional quality. It takes time and practice to develop the skill set to be published.