Important point, the Radian is VERY well known and highly respected plane from a known company with great support. You can get information from the flying community and you can get parts if you need them.
The Dynam Sonic looks similar, is almost completely unknown, from a company that has put out some pretty shoddy planes in the past (I have personal experience with 2 of them). There's a good chance you will get no service and trouble getting parts if you need them.
The Radian will have decent if not exceptional electrics. Spektrum gear is well known and accepted.
The Sonic electrics are likely to be bottom of the heap. If something goes wrong....it'll likely to be unfix-able. If you try to sell parts (like the transmitter or receiver if you upgrade later, no one will want them.
Short answer, go for the Radian, it will be worth the extra cost. If you got to go cheap get a Hawk Sky or a Bixler, both are copies of the Easy.
Get the Radian later.
Having said that I personally would not recommend either as a trainer for someone with no flight experience. You WILL crash them and they are NOT sturdy. Gliders are made light to fly on thermal updrafts, not be abused by learning pilots.
Get a Multiplex Easy Star instead. If you don't ruin it completely you can even sell it.