You will have different risks on both.
Motorways
faster vehicles around you, drivers play far less attention to mirrors.
Drivers may not change lane fully when overtaking a bike.
Safer asphalt (less hazards) fewer potholes. you can see further away on a motorway meaning easier to take evasive action if required, smoother bends which is nicer for rookies.
Country roads
A lot more fun to ride as they have more bends.
more hazardous, sand, gravel, crazy sheep, dumb pheasants etc.
You can find a car on the wrong side of the road when you come out or halfway a bend.
narrow road means shorter distance to eventual hazards
Riding alone on a country road can make you "invisible" due to sun glare, confusion with background (trees, bushes etc).
Unless you're familiar with the road "reading" where the bends lead is harder.
Road markings are more important than motorways to allow you to understand the layout.
Which ones I prefer?
by far the country roads
You can have more fun at 40mph on a country road than 90 mph on a motorway.
Something very important, in my opinion, is slow speed control. Learn how to make U-turns, left and right tight turns, left and right 90deg turns from stop.
Control of a motorbike (whatever the size) at slow speed will mean better control at higher speeds.
Something to avoid (no particular order):
road markings
metal covers
ride too close to the middle of the road or the verge
being anxious (if this ever happens, stop. get out of the bike, take a few deep breaths and resume).
Never think, "If I keep going I'll be there on time." Rush will be your worst enemy.
To keep in mind:
Where you look is where the front wheel will go.
When tired, stop and recover.
If you can't avoid a pothole don't panic. Isn't as bad as you think.
The friendliest thought is "I could have gone faster there..."
Most important, riding a bike is about having fun and arrive safe.