There is only one PSR Workstation still being produced and it is called the PSR 9000Pro. The 9000Pro has 76 Keys versus the Tyros' 61 Keys. The Tyros has NO Sampling Capability, No Expansion capability and thus it is not considered a Workstation. The Tyros' Sequencer has a Midi Timing Resolution of 1920PPQ versus the 9000PRo's 960??? I think. It could be lower, like 480PPQ. (The more "Parts Per Quarter Note" the more realistic and natural your recordings will sound). So the Tyros has the advantage in that area. The Tyros has more WAV ROM, "96Mb to be exact" than the PSR 9000Pro, which has 48Mb. The Tyros is will have more "realistic" sounding voices compared to the 9000Pro although the 9000Pro has many 'excellent' voices too.
So it depends on your needs. What you want out of a Keyboard. You say "Workstation" in your post. So the 9000Pro is the only Workstation Arranger from Yamaha that is still in production. Price difference is not that much. Both can be had for around $3,000 [1,892 Pounds Sterling]. You will have to shop around to find the 9000Pro for that price although it can be done.