Solely depends on what you do with your objective choices. 1k sons can camp on objectives in the open, relying on their 4+* save to see them through the fire. Rhinos act like mobile bunkers in this way, use them to deliever the sons to the objective, then to block fire lanes from guns that could harm them with ease.
Plague marnes can endure massed fire from small arms fire, with T5, a 3+ save and a feel no pain save, they become hard to stop/bring down. Again, a rhino is a must as it can be used to block fire lanes to them.
The majority of armies at 1000 points will struggle to bring enough big/assault guns to stop any of your choices. However, I advise away from a large 1k sons unit, as its such a drain on points, and importantly, space.
Keep your doors open is what I say. I prefer 1k sons over plague marines on the sole basis that marines with a icon of nurgle is much cheaper and can do the job almost aswell...AND can bring a heavy weapon. 1k sons bring you something that no other unit does, massed anti-meq firepower. You'll have to be careful with them though, SnP really hinders them, especially if their rhino is blown up. So they need to have their chosen destination picked carefully.
10 plague marines is over kill. 7-8 will do. saving you 40 - 60 points. Thousand sons need to be at least 9 strong, just because you don't want a job half done.