Welcome to the forum. This boating lark is all decisions, isn't it?
Assuming you don't want to add radar at some time in the future, the Standard Horizon may be your better choice. Theoretically, the CP-500 will take a radar input, but SH don't seem to be selling their radar antenna yet, and there's no online info on what it will do (eg will it do a radar/chart overlay or have MARPA, arguably two of the most useful features to have).
Leaving that aside, the CP-500 has a bigger screen (12.1" vs 10.4") - it doesn't sound much bigger, but it's about 35% bigger in area. With chartplotters, screen size is a major factor in user satisfaction. The CP-500 is also AIS ready and has a 3-year warranty (vs 1 year for the Lowrance). There are lots of offers available on the CP-500, and I see that JG Tech are listing it at £1199 with a free HX500 handheld VHF and a half-price MegaWide chart offer.
If radar is a requirement in the future, you'd need to check out just what the CP-500 will be capable of. The Lowrance 9200C is radar compatible and will give a radar/chart overlay but doesn't have MARPA (remember that an accurate heading sensor is needed by any system to give these features). But the Lowrance radome is perhaps a bit ugly, and costs about £200 more than the stylish Raymarine radome, which means that the Raymarine C120 with radome would only be a few hundred more than the Lowrance 9200C with radome - and in that case the Raymarine would be worth having for its larger screen, superior user interface, MARPA and AIS compatibility.
I have a CP-1000 and am changing to the CP-500. The SH units are easy to use. IMO its fantastic. And I say that even though I have had a warranty fault. They are excelent to deal with and having the 3 years is a comfort. I was glad I got the largest screen I could when I got the 10" and am looking forward to the 12". You really cant have a screen that's too big. (apart from space to put it I suppose) When I am planing and its rough the boat is all over the place, its quite hard to be working the zoom on plotters, with a big screen you dont need to so much as you can have more there.
The C-Map charts are fab, and the zoom is very good.
All IMHO of course. I have not used the other so I cant say how they compare, all I can say is I don't think buyers would be dissapointed with a CP-500.