Hi!
In my humble opinion, it's better to get a 6V panel. I didn't have the chance to try out 5V version, but their Voltage will most likely drop too much under load. How much will the drop be? To answer this question you need to know two things: 1) the current that the solar panel can deliver and 2) the current that the load will draw. There's also a third factor, which is the tolerance of the load; by this I mean how does the load tolerate Voltages; for example iPhones seem to be strict on Voltage levels, whereas a small Samsung mobile phone is happy with Voltages as low as 3.8V instead of the full 5V.
Usually the seller or the manufacturer specifies whether the Voltage is open circuit or under load; if neither of them do that then there's no way of knowing other than trying it yourself. Depends on who you buy the solar panel from, but usually you can send a question to them and then hope to receive an answer which reflects reality.
So far the only rule of thumb I discovered while playing with my small solar panels is that the Voltage drops under load. By how much, that depends on a few variables and conditions based on which part of Earth one lives and what kind of loads one wishes to power.
I underlined the word 'small' on purpose to highlight that I only fooled around with small solar panels. Be advised, that you could use a high performance solar panel to charge your 5V gadgets, but if we're talking about 2 A current or more then controlling Voltage is more critical because the drop will be less. As far as I'm concerned I'd plug my 6V/600mA panels to the phone directly knowing that it won't do any harm.
