From what I recall - no distinguishable difference, they are sent to wherever CX needs them.
The A350s are heavy workhorses in general and flown to almost anywhere within the network - I recall seeing an A35K on HKG-KHH (1 hour hop) after a 16 hour mammoth journey from JFK...
Though from what I recall, following dest are currently A359 specific:
Europe - MAD/BCN/MAN(Load ain't great so...)
NA - SFO gets the poor A359 whereas UA dumped capacity into the market..
JNB/CHC for dest. across the equator.
For within Asia - SIN sees high density of A359, NRT and KIX gets regular A359 and BLR are A359 specific, RUH will join BLR once launched in Oct.
Current timetable states that PER and BNE might see A359 again when freq. ramps up in NW24/25.
Oh as a local enthusiast I remember plenty failed CX pre-COVID long-haul trotting such as BRU/CPH/DUB/LGW/CPT/SEA sees the smaller A350 throughout their live...
For A35K:
For Europe FRA/AMS/ZRH
For NA Most East coast dest. are currently A35K only (YYZ/ORD/BOS)
For down under, AKL
I should mention that JFK/LHR/SYD/MEL are a mix of everything - 77W (Both 3/4 class config), A359 and A35K - while YVR are 3 class 77W only (Cuz for obviously everyone flies CX instead of AC whom can't get a 787 filled)
MXP and CDG sees 77W, both 4 class atm.
LAX are 77W only - but both 3/4 class config are deployed.
The ill-fated IAD sees A35K at first but towards the end only A359...
The A350 also flown everywhere across Asia if they're needed - BKK/KHH/TPE/HND/NGO/ICN/PVG/CGK all seen CX A350 at some point...
In general most destinations in CX network (Long-haul or Short-haul) sees A350s some point in their live, so just grab whichever one you like better