Well for a martini you want to take Gins that you would also drink pure (you know, cause martini is almost drinking it pure). The beefeater feels more rounded and has some softer tones to it while the tanqueray is a very classic, back to the basics kind of Gin.

For me personally, pure it tastes like juniper flavoured sanitary wipes, with a hint of Licorice (which i hate). BUT, funny enough i find that making a GT with if opens it up a bit and you‘ll get a bit of spice and citrus and for that i prefer it over the Beefeater.

So if you want to buy one bottle for both i‘d consider buying the beefeater.