Valve cover gaskets are on the top or side of the engine, depending on the design and when they start leaking, they tend to soil the exhaust manifolds, which are one of the hottest external parts of an engine. That smell is oil vaporizing. It's not uncommon for engines with some age to leak from several areas, including the valve covers. If you can smell it, then it's a leak significant enough to warrant a repair.
As others have stated, the leak will slowly drop your oil level in the engine and if it gets low enough, it will starve the engine of its vital fluid. Engines need oil to protect rotating and moving parts from direct contact with each other. It also serves some purpose as a coolant. The engine oil needs to be kept clean and changed frequently to ensure these parts don't wear prematurely.
It's kinda cool, actually - some of these parts are suspended by a film of pressurized oil. The two parts never really touch each other, although they appear to be in direct contact. Thousandths of an inch or less separate them, and the oil takes up that space, allowing the parts to move and "float".
Control arms and struts are both front suspension components. They connect your car's chassis to the wheels, essentially and allow for steering and independent movement.
Typically, the control arm would be the more serious concern because it is directly responsible for making sure the wheel and steering components stay connected to the car's chassis. If a control arm (or ball joint on said control arm) fails, you may lose all control of that corner of the vehicle.
A strut is a shock absorbing component, similar to a shock expect that it it combines a spring and shock into one unit, and also provides in some cases a fixed attachment for locating the control arms to the frame. When a strut fails, it can either be due to a broken spring or (more likely) simply a leaking shock absorbing component. If it is just a leaking shock absorbing component, you will notice the car riding like crap and not sticking to the road well, soaking up bumps, etc.
Both are safety concerns - depending on the make and model, if you had to pick one to do first, it would be the control arm.