it's said to be good at dusk, so if you took the 1700 slot and stuck around a while (I don't think you get thrown out) you might see that.
Otherwise... the sun moves across during the day. In the morning it will be shining on the buildings upriver - the City, Westminster and the South Bank. In the afternoon it will shine on buildings downstream, but there aren't many interesting ones except the Tower.
And of course it may not shine at all, this being London.
I'd go for the morning view myself (in fact I have). If you're taking pictures they will be through at least one pane of glass, sometimes two, which can dull your photos - it's not very well set up for photography. There are a few points on the upper level where you can poke a small camera through gaps in the glass, or hold it up over the top of the panes, though.