The BTR5 has a separate USB control chip (Xmos XUF208) and can therefore handle higher bit rates and bit depths when used as a USB DAC. The UP4, UP2, BTR3K, etc. are limited by the USB capabilities of the CSR bluetooth chipset. The BTR5 also supports DSD format.
However, the Xmos chip does consume additional power, which is one of the reasons for the lower battery life.
In the UK, the BTR5 is actually cheaper than the UP4 (£90 vs £100) when it's in stock. The only reasons I see to buy the UP4 over the BTR5 would be battery life and the wheel controller.