Which Turntable would be better?

NO! Just get a good cartridge - an AT740ML is an excellent recommendation! I have the KD-650 in my collection of TTs. These vintage Kenwood's have excellent DD motors (lots of torque) mounted in substantial cast resin plinths. Unlike the Thorens' low torque motor in an MDF plinth. For TTs mass and torque are important! 

What phono stage will you be using? That is something to consider investing in!

BTW, I have a Thorens 160 with an SME III arm. But I prefer the KD650 with the stock arm.

Thank you for the input.

I haven't done any research yet on a suitable cartridge. I believe the one on the TT now is an old AT Z100.

The preamp is a Kenwood Basic C2 and the amp a Kenwood Basic M1 into a pair of Dayton Wright LCM-1 speakers.

BTW the Linn Basik is mounted on a Honduras Mahogany base rather the the plastic covered plywood that came with the KD500.

I love those vintage Kenwood tables. I would keep it. 

Keep the turntable, it is an excellent player.  I used to sell the KD500 and also the same but with a Kenwood  "S" shaped arm.  I owned the latter one.   When moving from one home to another, it fell off the back of my Toyota pick up.  I was in the road crying, as I picked up all the pieces.  You will never get the same PRAT  with a belt drive that you get with that Kenwood.  It's a timeless classic.  I would upgrade the cartridge and perhaps the phono stage.