I have both the AirSpyHF+ and the KiwiSDR.
The AirSpyHF+ does sometimes produces images from MW stations, although the recent library updates seem to have (mostly) fixed/reduced that. The AirSpyHF+ tunes some of the VHF band, FM and VHF-Hi if that is of interest to you (it does not do Vhf-Lo or 6 meters). Right now the AirSpyHF+ mostly sits on 14230, decoding SSTV. The AirSpyHF+ may be slightly more sensitive than the KiwiSDR, but not by much. If you want to be able to record sections of HF as IQ files for later use, such as the overnight SDR recordings I do, then this is where the AirSpyHF+ would be better than the KiwiSDR.
The big advantage (to me) of the KiwiSDR is that you can easily put it online to share. And there's zero setup or software issues, you use a web browser. It "just works". And, I find it is great for doing antenna work. I take a tablet outside with me (you can use a laptop also) and can see the effects of my antenna tweaking in real time. I've also been known to take a tablet downstairs to access it when cooking, and I can use a browser on a computer in the basement workshop to listen as well. The ability to use it from anywhere in the house is (to me anyway) huge. It does however suffer from images if you feed it too strong of a signal, there is no attenuator. At night, my 670 ft sky loop overloads it, and I see MW images throughout HF. If you have a more modest antenna, that will likely not be a problem. I now have my 350 ft LoG (loop on ground) connected, and have zero overloading issues. If you connect a smaller HF resonant antenna, say a 43m dipole, I doubt there will be any overloading issues.
This is where a (much more expensive) SDR like the netSDR shines. It has multiple bandpass filters, and does not overload or produce images. But we're talking an order of magnitude more expensive.
If I had to choose between the two, I would probably pick the KiwiSDR, unless I did not have another SDR *and* being able to record overnight was important, then I'd pick the AirSpyHF+.