There are three types of panels to consider; Polycrystalline (Poly), Monocrystalline (Mono) and Thin Film. They all have slightly different characteristics and perform better in different scenarios. If space is not restricted, thin film is the ideal choice. It performs better in low light conditions (cloud cover or towards the beginning or end of the day), resulting in greater energy generation and therefore higher payments through FITs, along with increased savings. They are more tolerant to higher temperatures, which improves production in the summer months. Poly and Mono require less space per kWp, resulting in more capacity from a smaller area.
The price per watt does not vary hugely between the middle range of each type of panel, i.e. 10kW of poly, mono or thin film will cost a similar amount. The high-end mono panels, which offer the most wattage in the smallest area do carry a premium; usually only worth paying if space is limited and the main driving factor is to install as much capacity as possible. A bigger price range can be found between different manufacturers irrespective of panel type; contributing factors include the output, origin, brand power and warranties.