Without the part numbers we're guessing but the broad classes for efficiency are amorphous < polycrystalline < monocrystalline < hybrid amorphous.
The cell efficiency goes up to the right of the list. The module efficiency measures how tightly the cells are packed together and how lossy the interconnects are and how transmissive the glass is. So two modules made from identical cells can have different module efficiency ratings if one uses dodgy glass and spaces out the cells so that the module is physically bigger than a smaller one of the same power.
Sharp modules come in different efficiencies (the ND170 and 175 are the same physical size but have different power outputs due to cell grading).
Cell grading also plays a part in the overall efficiency of a module and its cost. For best output a module must have all of its cells closely matched (as the current output of a cell string is limited to that of the weakest one in the string). So, modules made from close tolerance cells (+-3%) produce more power than modules made from randomly chosen cells (+-10%), as the chances are that a module will have a -10% cell in it somewhere and that will limit the whole module power. Worse, several modules are needed in series for a grid tied inverter so the power of the string of modules will be limited by the power of the weakest module in the string.
The other factor is cell behaviour with physical temperature and colour temperature. STC is measured at one light intensity 1000W per sq meter, at a cell temperature of 25'C and direct daylight colour temperature. But while mono cells perform the best at this test, amorphous cells perform better at partial light and under partial shading events and tri-junction amorphous responds to three peaks of wavelength (more in the blue range) so performs well under overcast sky with a very high colour temperature.
That's the theory behind the hybrid HIT cells from Sanyo - they produce more total output for a given STC rating because they produce more power at low light and high temperature. The STC test assumes a cell temperature of 25'C. Not realistic when the black cells sit in the baking sun for hours. The hybrid amorphous HIT cell has a lower power de-rating gradient with temperature so produces more power when at 75'C than a pure mono cell. The standard HIT cells claim a 10% higher output than monocrystalline cells in these conditions. There's some charts and stuff here:
http://us.sanyo.com/Solar/SANYO-HIT-TechnologyThere's a movie showing the applications of a new double sided HIT module range that claims 17-25% increase over the standard single sided HIT modules as they can absorb diffuse / backscattered light from under / behind the module. Obviously, this lends itself to mounting situations where the back of the module is exposed to some light (so use in bus shelter roofs, porch roofs and tracking arrays). My own system would benefit from double sided modules as I have angled panels on a flat roof and a home made porch roof thing over the patio doors formed by 370Wp of PV. With a white painted wall and light coloured paving under it, considerable backscatter light could be achieved in my set up.
Is it worth the extra cost? Depends on how keen you are to wring every kWh out of a small space and how deep your pockets are. Are they over egging it with the "1.25 PPF"? Most probably, unless you're looking to use HIT double sided modules and build an open plan bus shelter with a white floor.
