I have not stayed at either (only lived for a long term stay at the Konventa Seta). The reviews for both are fairly good - and the Radi un Draugi is pretty popular, so it must have something going for it. Its location is excellent and it always appeared to be well managed to me.
The location of the Centra is great, too, so I guess it's a toss-up between the two of them.

If you'd like things to be a bit more quiet, you might check the Hotel Gutenbergs, which is located in Old Town, too, but a bit away from the buzz along Grecnieku and Audeju ielas.
It is tucked away behind the Dome cathedral.

If have no commercial or other interest whatsoever in this, but would recommend you take a look at

http://www.viariga.com/pub/?id=1&sub=1&h_loc=1

for a travel agency that often offers good rates at local hotels.

Riga Inyourpocket also might offer favourable rates through their invalubale website (try to purchase a current edition of their continuously updated guides upon your arrival. They are for sale at most hotels and kiosks such as the Narvesen chain. They have great information on places to see, dine, shop and have fun and only cost a really small amount of money).

http://www.inyourpocket.com/latvia/riga/en/

Hope this helps (although this is not first hand information on the two hotels as you might well have liked better).