Thats quite a difference! Did you swoosh them both through the water?
Take a look at the 2 prongs on each and they should both be parallel to each other, ie straight? If the prongs aren't straight it will be wrong!
Otherwise take a sample of some water from one of the sources, to a nearby local aquarium place and get them to check what reading they get and 'hopefully' it will agree (near enough) with one of the meters?
The zerowater one is nearer the figure correlation that beanbag mentioned so maybe?
If you want to change the water in the tank before you can get confirmation which is right then I would use the Zerowater meter as those figures are higher, so if it turns out the lower old meter is right you can just add more GH/KH+ which is easier (and a lot less wasteful) than trying to reduce it! As the zerowater figures correlate better to GH figures etc that is probably the most accurate!
edit - sorry, just realised beanbag TDS/GH correlation is on another UK thread! 1 GH is very roughly USUALLY approx 20 TDS.
My Zerowater jug arrived today with a free TDS meter. I had already bought a meter a couple of weeks ago so I decided to compare them. These are the results:
Free meter Bought meter
Tap water 317 227
Filtered water 0 1
190L tank 510 but dropped to 484 334
Nano tank 384 but rose to 410 306
Knowing nothing about these meters I don't know how reliable or accurate they are but these readings are wildly different.
I suppose the important thing is that I'm getting pure water from the filter so once the cycle is over and I do a big water change I'll have the water I want.