I've used both.
The Keysight is a very basic scope. It has a UI derived from the 3000 series, but it feels cut down, cramped, and less responsive that its big brother. Annoyingly, it's also subtly different enough in layout that even though I use a 3000X every day, it still doesn't feel familiar. But it works just fine, and if you get on with the Keysight UI then of course it'll do a good job for you.
I bought my own SDS804X-HD last week. First impressions are that the user experience has a lot in common with the Keysight - also a bit cramped and fiddly, and if you can stretch to a model with a larger screen you won't regret it. But for audio work it's definitely a better bet IMHO - the 12 bit front end will make seeing small signals, and quantifying the noise on them, much easier.
You don't want 200 MHz for audio, it'll just increase the noise floor for absolutely no benefit whatsoever. You don't want 70 MHz either, frankly. Expect to use either scope with the 20 MHz bandwidth limiter switched on most of the time, and don't waste money on anything other than the lowest bandwidth version of either scope. Even digital ICs won't need the bandwidth if they're 'vintage'.
There's a lot of discussion on the forum about the Siglent right now, plenty of useful help and support provided you can overlook the occasionally defensive and abrasive comments. You won't get much community support with the Keysight because they just aren't as popular with hobbyists.
Can you get a demo of either? It may just come down to which scope you prefer the feel of.