RX240 and RS360 will perform pretty similarly. RX series is the XSPC 'thick' design and the RS is the 'thin' of their older models. The RS is pretty much outdated, but a decent rad. Newer models are the EX (and maybe an AX line?) and offer greater fin and tube density in an RS 'slim' package to perform similar to the RX from most I have read.
An easy way to figure out a radiator's performance is by using the coefficient table that I have listed in the watercooling sticky. You basically calculate the total radiator volume in millimeters and multiply by the cooling coefficient factor of 0.00023129193:
I've put together a chart that defines some different cooling properties of common radiators and their cooling potential based on total volume in cubic millimeters (mm^3). The list below is ranked based on thermal coefficient; essentially a product of a radiator's heat in watts for a 10°C delta-T with 2000rpm fans divided by total radiator volume to achieve the average cooling potential of all 15 radiators listed and reviewed by skinneelabs.com/water-cooling-radiators.
This can also be used for a very quick cooling performance estimate for total volume of a radiator based on the average thermal coefficient:
[volume LxWxH in mm] (LxWxH) x 0.00023129193 = Watts dissipated for 10°C delta-T (estimated)