515r or 520t for new alarm or both?

Hello, been about 10 years since I have installed an alarm.  Purchased the Viper 5902, installing in 2007 Toyota Tundra Pick up. 

Question?  Which one is better and why?  Or should I use both the 515r backup battery siren or the 520t back up battery for the whole system? 

I am thinking the 515r is the better choice, but would like to here some opinions and why? 

Thanks Andy

Never used the 515r but i've seen dei's back up battery siren in action it's more effective than the battery i would say, until it's screwed out and tossed far away by the would be thief, one time i removed one from a car on a saturday put it in my work drawer to run out for the weekend, come back to work monday morning and opening the tool cabinet what do ya know, it's still blaring, lol, ended up throwing it away cuz it got annoying! LOL!