You can think of HSPA as an upgraded version of UMTS which gives you the ability to use higher data speeds. HSPA is like a feature of the 3G network which improves data speeds by giving you a channel with higher capacity. It has own transport layer channel and 3 physical layer channels.
HSPA uses better modulations in good radio conditions to deliver higher speeds.
HSPA is part of the UMTS and it and it is only for data.
UMTS uses for data the lower speed R99 data channel but it ca also carry channel switched voice and video.
Operator offer both UMTS data and HSPA data for backward compatibility, not all handsets support HSPA.
A user cannot choose which technology to use. The user equipment (the handset) automatically selects which technology to use according to the radio conditions and the network settings. You can only choose between 2G and 3G or automatic, this is the only setting I have seen.