There is no definite answer to this as both have their advantages and disadvantages. VHF has traditionally been less expensive to buy than UHF due to the components and the larger batch production by individual manufacturers, although we are now starting to see the price of UHF systems fall.
The operating distance between the transmitter and receiver of a VHF system is superior, as is the ability to maintain a signal even if direct line of sight is broken. This is due primarily to the longer wavelength of a VHF signal compared to smaller UHF wavelengths.
The main disadvantage is that there have been more units of VHF sold, and so the chances of clashing frequencies with a nearby user are far greater.
The airwaves are a lot less crowded with UHF frequencies since they are much higher frequencies thus making them popular in built up areas and busy broadcast areas. These are also popular for touring groups because of the larger amount of frequencies available. The ability to select multiple UHF frequencies is another benefit favored by touring groups because they can avoid using frequencies that are already in use locally.