Not a whole day surely?
Was thinking of Llandegla myself for Sunday but as you say, the forecast...
Marin.
Quite a lot of fireroad climbing (15-20 mins near start and more further on). Seems to spend the climbing too quickly but some very good techy & rocky type singletrack. Can be slippy (wet rock) when wet. One of the more exciting runs I've ever done was a wet day on the Marin on my old Genius with it's original non grippy 'Scott Cougar' tyres. A lesson in balancing the need to brake against the need to stay on the track

but it's fine with decent grip.
The final dowhill sections are quite simply fantastic and you can just do the initial climb followed by the final downhill if you desire.
Penmachno.
Still a fair bit of climbing but seems like less than the Marin. Not as technical as the Marin but lots of good fun pumpy 'attack position' singletrack. It's very good when dry however I've always found it quite miserable in the rain. Parts of it seem very open & remote so if the weather's poor carry an extra layer. It's possible to cut large parts of it out if the weather gets really bad ... if you have a map (and thereby hangs a tale). New extended section supposedly open now but haven't done that yet.
Llandegla.
Fun with berms & rollers. Not really rocky or technical unless you're going for it over the jumps

Black sections optional but not too demanding unless you do them at speed.
I would say all three cope well with rain long as you don't mind the wet arse.
Marin & Penmachno are close together and have the Conway Falls Cafe inbetween

Llandegla has it's own nice cafe but is a bit of a drive from the other two.
Unless super fit the Marin will take anything from 2 - 3 hours depending on conditions, snack breaks etc.
Penmachno probably 2 hours, maybe less.
A round of Llandegla including the black bits perhaps 2:30 - 3 hours?
If you're driving 200 miles before starting you might run out of daylight. Start early.
Forecast could be showery, windy & cold or not too bad depending on which forecast you believe.
Better later & inland.
http://www.mwis.org.uk/mountain/SD.PDFhttp://www.metoffice.gov.uk/loutdoor/mountainsafety/snowdonia.html