There was a bit of discussion about this very issue at GPT that may be helpful, see http://www.greenpowertalk.org/showthread.php?t=1974

I have some old ARCO monocrystaline panels that are probably going on 25 years old and still working.  I purchased these 2nd hand, so I don't know all of there history.  

In my opinion, if I were buying, and in fact when I have bought, new panels, the following are items that I look for in no particular order:

1. mono-crystaline silicon panels, if not mono-crystaline which I prefer, then poly-crystaline.  These materials have survived in existing panels for long times and functioned.  As noted above, some of my mono-crystaline panels are 25 years old, and the oldest working panel from ATT Bell Labs is now over 50 years old.
2. eva encapsulent. With the exception of panels that have been under a concentrator, this appears to hold up well and has stood the test of time.
3. a thicker backplate as opposed to the thin sheets of plastic tedlar now used on many.  I have seen some of the newer panels with the thinner sheets of tedlar (white plastic) backing that have had that layer cut or torn, generally from mishandling.
4. I prefer companies that have been in the business a long time, offer long term 20 - 25 year power warrantees, whose products have stood the test of time and not changed much in there design.  
5. I also look for panels with a weather tight junction box on the back with standard knockouts, because over the years I have moved things around a few times, and I find that these tend to hold up well.  Potted boxes on the back with wires sticking out tend to fray and break where the wires enter that plastic box.
6. Additionally, I look for an appropriate certification, UL, or CE because if the panel is to be part of a grid tie system, or if the installation is ever inspected by an insurer, I worry this may be important.  
7. A sturdy aluminum frame.  The panels will be exposed to severe wind, weather, and elements over the years.
8. tempered glass that is not too thin.  shipping, mishandling, dropping stuff, hail would I believe likely be better handled by stronger glass.

I would lump most of the "silicone" manufacturers into a single pot.

SunPower has some of the highest efficiency (consumer) panels on the "common" market.  However, I think they sell their panels in somewhat odd voltages.  They are rated around 40 volts.  I.E.  Not recommended for a 12V system.  Wasteful at 24V, and perhaps ok for a 36V system.  You would very likely need a MPPT device to get the voltage to the desired point.

"Efficiency" is merely a measurement of how large a panel will be for a particular amount of power generation.

If you have a large, south-facing roof, and plan on putting flat panels that only will cover a portion of the roof, then efficiency is less important.

However, if you are investing in a solar tracking system, and/or have limited space, then efficiency may be more important.

I have a set of "used" Siemens panels.  I believe they were originally installed on a mobile RV system, and their power generation SUCKS.  

I don't know if these panels have suffered years of damage, or if Siemens panels inherently have problems.

My next set of panels will either be NEW, or UNUSED.

Many people are highly suspicious of "glass" based panels, "thin-film", or "flexible" panels.

There are a few companies coming up with a foil-based panel that report that their production methodologies are far less expensive than other types of panels.  However, I have yet to see the promised $1 / watt panels hit the market.

You will find that the cost/watt decreases steadily with the increase in size of the panel.  So, a 10W or 20W panel might be $10-$20 / watt.  But, a 200W panel might only be $4-$5 / watt.

There are also a few more "radical" designs.  For example, cells or panels with tiny holes in the cells to allow ambient light to pass through (skylights, partially obscuring windows).  Another unique design can essentially be installed with regular roofing.  A third unique design is a "double-sided" cell.  I am a bit puzzled on the benefits, but perhaps there would be benefits for generation on cloudy days, and/or certain applications where the panels are mounted with good rear visibility.