is there a advantage if my modem is DSL to having a dual band router?

DSL is just the connection on the line to the router.  Dual band is the wireless connection around your house.

Generally dual band means you have a slower but longer range 2.4GHz connection with a faster but shorter range 5GHz connection.  

Really it depends on your house layout, devices connected and what you use it for.

Generally dual band is better if you have devices that need faster connection.

Also what speed is the DSL, a 1mb connection it probably doesn't matter at all but with a faster connection it may make a difference.

Dual band is in reference to the Wi-Fi capabilities of the router.  It has nothing to do with the broadband technology you're using.

Dual band router supports 2.4Ghz and 5Ghz carrier frequency for Wi-Fi.  Generally speaking 2.4Ghz is the preferred carrier frequency to use because its effective range is larger than 5Ghz.  However if you have issues with interference on the 2.4Ghz band than 5Ghz would likely work better for you.  2.4Ghz is the same frequency used by other consumer wireless devices such as baby monitors, cordless telephones.  You can also get interference on the 2.4Ghz band from other Wi-Fi routers that are nearby, say if you lived in an apartment building.

The FCC recently opened up the 6Ghz band for Wi-Fi use, so soon you will see triple band routers on the market.