I was also wondering the same. I've been following discussions on Spectrum Analyzers for a while, and while the Rigol DSA815 used to be the community's darling for a long time, I've seen a lot of love for the Siglent units as well, so at this point I don't know what would be the "best" purchase anymore.
In my case there's the additional challenge of limited options and higher prices. We have a local Rigol distributor and that's it. They bring the DSA and RSA lines, but they only keep stock of a couple of models, and the rest must be requested on-demand with some relatively long wait times because they bundle client requests into shipments which don't happen that often (low demand for this kind of stuff means less options and higher prices, unfortunately).
Right now they have the RSA3015N in stock but it's like 2700 USD (after taxes), so quite a bit outside of my budget (I was thinking around 1500 USD).
The Rigol DSA815-TG is something I could buy for around 1350 USD from them (again, after taxes) so it's probably the option that best aligns with my budget.
My other option would be bringing the Siglent SSA3021X straight from Amazon, which would result in a 1750 USD purchase (after taxes and shipping), which I can still afford, but it's $400 more than the Rigol, and $200 over my "expected" budget goal. As far as I know the Siglent has lower phase noise, lower noise floor in general, faster sweep, and higher frequency range than the Rigol so I'm still considering a valid option. Is that worth the $400 difference? That I don't know. I guess I could wait until there's an Amazon coupon or something for the Siglent, lol.
While I don't necessarily need to measure EMI emissions for any mission critical job, I'm interested in designing testing and tuning RF filters, doing electromagnetic emissions testing, and performing frequency-domain analysis in general, and maybe the Rigol would suffice.
I know the OP mentioned the DSA line in their opening statement but didn't list the DSA815TG between the options they were considering, but between the DSA815-TG and the SSA3021X (not Plus), which one would be a better purchase? (considering the pros of the Siglent: better performance, higher frequency range, and the cons: $400 more expensive, brought from abroad so less chances of warranty/tech support)