I think I'd go for a BS nightlatch on each, which you will use most often. and a BS mortice deadlock as well, although I expect you will hardly ever use it. The mortice deadlock is cheaper to buy. The red-cased 5-lever Union deadlock comes out well on tests and is good value, widely sold. Easy to change if you lose a key or move.
Two locks are usually fitted one-third up from the bottom of the door, and one-third down from the top, this spreads the load of an attack. The nightlatch is often fitted as the higher one, because it puts it out of reach of small children who might otherwise open the door and wander into the street.
If the downstairs door opens onto the street, and is not shared, I think that's the one you need to take more care over, because if an intruder gets through that, he will be out of sight inside the building.
However, a shared entrance door is not secure because you can never be sure that other users will lock it, it might be left ajar when they go out for a fag or are expecting a parcel, or let the cat out.
When you are at home you don't need to keylock the nightlatch, so you can let yourself out by turning the inside knob, for example in an emergency, so this is the one you will use most often.
You might like to fit a viewer and a doorchain while you're about it. The type illustrated here is more heavweight than some.
A carpenter or locksmith usually fits door furniture more neatly than an amateur.
If you are still in doubt, look for a local accredited locksmith business who will be able to make a recommendation and show you the range, they are likely to charge more for fitting. The people who advertise as "24-hour emergency locksmiths" are sometimes unskilled and high-charging.
You home insurance company will have a leaflet or webpage called "Minimum Standards of Security" which I expect will specify BS locks and security of windows, have a read of that.