Which one did you prefer the sound and touch of? That's the one to buy. You might need to go back and play them for fifteen minutes each again to find out.
I'm pleased with the U1 I bought new five years ago. I never tried a P121 so can't make a comparison though.
However I googled "Yamah p121" and came up with this:
Yamaha P121-nt European 121cm
This is a fantastic buy.
Introduced in 2003 this UK-made Yamaha is based on the Professional U1n and V124nt to the extent that we just cannot find any technical difference other than the style of case. The performance in tone and touch seems no different at all and, as technicians, we have inspected and worked on the core components (frame, action, keyboard, soundboard, hammers and strings) and we can see no reason why there is such a difference in price.
Why is this such a good buy?
This model was introduced to broaden the range of 121 framed models. This piano is equally as impressive in overall build, component and performance quality as the latest U1.
If you have seen the many rave reviews on the professional U1, it would be tempting to find a second-hand one. However, this piano is certainly no compromise against the current U1 and more importantly, it is a brand new piano. The hundreds of imported late 70's/early 80's U1 models do not feature the latest improvements, whereas the P121nt does.
The P121nt is better than a U1 made before 1999 in design AND is lower in price than the U1 was then too!
Despite what many dealers may say (who realise the greater profit on a more expensive piano or indeed the used pianos) the U1n and V124-nt (exellent offer available) are hardly any different to a P121nt in their technical specification if at all, other than the cabinet, model number, brochure position, brochure description and price. The U1 and V124nt do however feature the soft-close fallboard (anti-slam keyboard lid).
You're lucky getting a good piano!
