They are different ;-)
In 2004 I found Poseidon a very good tool. But, it is a website, which means you need full web access to view it.
Being local from the Greek, I'd think it is very accurate. As far as I recall
it also gives wave data, which is helpful when cruising with a smaller boat.
UGrib in turn is a program you install on a PC and use it to watch GRIB Data you downloaded in form of very small files from the web.
I tested several GRIB file viewers, my preferred free one is:
Grib files come from several locations, a good one is:
FREE GRIB Files | Marine Weather Charts | Global Marine Networks
Where you can send a very short coded email
to and receive the required GRIB automatically back.
Remember, you do not need to buy a program to use these files. Zygrib handels them well.
If I where you, I'd use Poseidon if I am in an Internet
cafe or have full web access, or if I require more info than from the GRIB.
When on board I'd use Zygrib.