Switch on set of 6V battery powered lights from raspberry Pi
Assuming that the LEDs' specs aren't horribly misquoted, you will easily be able to find a BJT or power MOSFET to do the job.  I would still measure the current of your LED strand, though.

If you use a MOSFET, make sure not to fall in to the trap of selecting a FET with a gate-to-source voltage requirement which your system cannot supply:

|V_gs| > Vcc: with the Pi, this would mean you cannot use a FET with |V_gs| > 3.3V.  Otherwise, your FET may not switch on reliably (or at all).

If using a BJT, I would make sure not to drive the base too hard: this requires choosing an appropriate base resistor.  Check the GPIO current limitations for the Pi for further information.

Mechanical relays often require a higher voltage to operate (and higher current), and care must be taken to account for reverse voltage (using a flyback diode).

Solid State Relays (SSR's) operate at a lower control voltage, but have a higher cost than mechanical relays.

Both are good options when a transistor just won't do (especially in situations with high voltage AC), but this application will most definitely not require something that beefy.

Regardless, always check the datasheet of whichever method you select; relay or transistor.  Again, I would highly recommend measuring your LED strand with an ammeter, before trying any type of control.

I hope that this helps.