More time now . . . sorry, this is going to be a long one
OK, so generally, of the two, solar hot water panels are much more likely to give you an 'economic payback' ie you are likely to save more money than it costs you to put them up there within a reasonable period of time. They are also probably the best bet in terms of reducing fossil fuel use relative to the amount of resources used to make them. And they can be very cheap indeed if you are handy and can make them yourself - they'll be less efficient than the bought ones, but then you can make a bigger area of panel. (FWIW we had one that DH made from an old radiator, and it would regularly produce water too hot to put your hands in.) Often the main cost is likely to be plumbing it into your hot water system, particularly getting a special tank with two loops. But even then there are ways round that (eg two standard - & therefore cheap - tanks side by side linked top & bottom)
If you only have space for one thing, and if there aren't plumbing related reasons why not, hot water panels are generally I would say considered the best bet.
The thing with photovoltaic panels is that it is (ignoring potential improvements because of feed in tariffs) much more expensive than buying electricity from the grid.
BUT equally, most of us don't make every (or even most) decision in our lives on the basis of cost. Lots of people wouldn't buy battery eggs for example, even though they are the cheapest, and most people don't choose their car on the basis of what is cheapest. So, (to put it emotively) you might feel that even though it will cost more, you would rather have clean electricity that isn't going to trash the planet.
Having said that . . . the best bet first of all is to look at how much electricity you use and where you can cut it. It is always cheaper to reduce how much you use, rather than try to generate more power. Which sounds really obvious, but it is the part of the equation that tends to drop off the list as it doesn't involve anyone selling anything!
For reference, we're in Wales, and we can assume, on average, 2 'standard sun hours' per day in summer, and 1 'standard sun hour' in the winter. So if we have 1 KW of panels (costing say £5K including inverter to connect it to the grid etc) then we will get on average 1.5 KWh of power per day.
At the moment our electricity use runs at about that (2KWh per day winter, 1 in summer, so back to front to production, but since we're thinking of a grid connected system that doesn't matter so much). I would say our usage is very much on the low side as although we're living on grid at the moment its new for us & we've been off grid for years so we're in the way of keeping it down to a minimum.