One could give a very involved answer to this, since it is acutally complex due to things like stress concentrations, loading dirction, load type, etc.

The simple view - stress is the force (load) divided by the area that acts upon. The larger the load carrying area - the lower the stress. Adding a groove weld increases the area that will transmit the load.

S = F/A larger A gives smaller S.

Regards.

If the fillet welds are the same size. The fillet weld inside groove outside would be stronger. Not better though. If the strength isn't needed, its a waste of time.

With A single sided fillet weld, it would be very easy to cause failure at the root by bending.

Any weld opposing the 1st fillet weld will prevent that weld from being subjected to tension at the root.

Here's a third option, corner joint.
The joint is prepared by default for an outside fillet weld, if more strength is needed you add an inside fillet weld, unless you have an access problem.
Is forming and option?

Regards, Brad