This is not exactly a scientific comparison, but here are 2 polar diagrams, the first from the Kinetic SBS-1 using the BS1100 external antenna, and the second from RadarBox using the standard antenna mounted on a metal plate about 30cm square resting on a Velux roof window. The BS1100 antenna is slightly higher, but has a longer cable run which will result in some losses. Also, these polar diagrams are produced months apart (Dec 06 for the SBS and Aug 07) so the differing atmospheric conditions will have produced sightly different results. Although it is easy to say that coverage is better in one or other of the diagrams, the pattern of coverage is strikingly similar - note that the pictures have come out at a slightly different scale, so compare the polar diagrams with the coast line in each picture.
All in all, I'm very pleased with the performance of RadarBox, and in daily use I have found little difference between the two. The advantage of RB is though, when aircraft fly out my live coverage area, I can follow them via the network data :-)
I've tried to superimpose the two pictures below. The white polar diagram is the SBS-1 and the green diagram is RB. Note the 2 BAW RB network tracks (NW an SW) well outside either polar diagram - all non BAW tracks were filtered out on RB.
