"best" can depend on what your controller can handle. 260W panels tend to be "24V" ones and have a Voc (maximum Voltage possible) of around 40V.
If that is the case (check the label on the back of the panel) then connecting in series will put an occasional voltage of around 80V into the controller - that is fairly high and many controllers will not accept that. If that is the case, than connecting parallel is the only option (other than changing controller of course)
Connecting in Parallel with keep the voltage the same and double the maximum current - A 40A controller should be able to copy with a 520W PV Arrays current output, but again check the maximum current in allowed by the controller against the maximum current output of the controller (again, info on the label and remember to double current for parallel setup).
If you have the free choice between Series and Parallel, then pros and cons for both.
- Series is slightly more efficient as higher voltage and lower current = less cable losses in power transmission
- Parallel allows one panel to have shade without the whole array being affected
For your 2 x 260W Array, I would be inclined to opt for Parallel.
The real-world increase in performance of Series over Parallel is negligable, but if you controller can handle either option it will be dead simple to try them both and see if it makes a difference,
