Hi Chris,
I own a Flex (5000) and a Kenwood TS-590S and have to say they are both excellent rigs that perform beautifully. If I could only have one rig I would definitely choose the Flex platform since it offers more versatility than any rig I have ever owned. Once you have a panadapter it's really hard to go without one! That being said, the TS-590S certainly holds its own and in fact at this point I prefer to use it for all my digital mode communications and CW.
If you have narrowed it down to the Flex 3000 or the TS-590S you won't be disappointed with either choice. If you are comfortable with your computer and are willing to read the PowerSDR manual the rewards for traveling the SDR path are great. The issues with RFI problems are horribly overblown by those who have not bothered with installing a single point ground system for their operating bench. I run full legal limit power with my Flex 5K from 160 - 10 meters and have never had an issue.
For better or worse, if you are interested, I have some information about the software EQ settings for the TS-590S at the link below:
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www.w1aex.com/ts590s/ts590s.htmlGood luck with whatever you choose to run with!
73,
Rob W1AEX