The #990 is legacy base plus cab2 for $289.95, or so.
The #993 is just a battery charger holder plus cab2 for $209.95, or so.
If you can afford the $80 difference you should get the extra #990 instead of a #993. You will be getting a spare legacy base, that can be very useful for emergencies. The cab2 that is not being used can be seated in your layout's legacy base to charge while you are operating your trains. Rotate the cab2s to keep batteries charged.
Sooner or later your legacy base will fail or you will have a problem that you "think" could be caused by the legacy base. The spare base can help you isolate the problem. It can keep you running if you have to send your base in for repair. I had to send my legacy base to Lionel for repairs two years ago. Now I have to do it again. The serial port won't download the legacy software update so I can run the new LCS hardware. I tried 3 different serial to usb cables before concluding the serial port is the problem. I have a #990 and a #993. If I had bought the extra #990 instead of a #993 I would have been able to figure things out sooner. Now I must send the legacy base to Lionel for repair again. It is not summertime and it is not OK to shut my layout down now. I refuse to run conventional. I decided to buy another #990. The new ones are supposed to be more robust and it will give me a spare base. Also another cab2 to make recordings on.