The performance should be similar. I've heard that the 9555 draws slightly more power, but even if true it should be so little that it makes no difference (I've got the 9505A, and it takes about 0.1A @ 12V when charging).
The 9555 is able to send and receive SMDS messages, and while these aren't going to be used for weather this might be a feature you want. The older 9505A only receives SMDS. There are newer versions of the 9505A that can also send SMDS, but I don't know where the cutoff is.
The 9505A has a very clumsy arrangement of stacking connectors for charging
data interface. You will need an RS232
adaptor to connect the phone
to your computer. The 9555 has a USB port which makes data interface much easier. (Is this used for charging
too? I don't know.)
The 9505A also has a clumsy external antenna
connection. The 9555 looks cleaner, but I have not actually tried it.
Originally the 9555 had problems with XGate drivers, but it is supposed to work fine now.
For what it's worth, I use XGate with my 9505A and an external antenna
. It works very well.