There is quite a difference between the two. Robert is right (txairgunners). Other than the trigger on the B-20 which is far superior there is the following:
B20 = clone of Beeman R-9 -or- Weihrauch HW95 –or- RWS320
B18/19 = clone of GAMO 220/440/890 (Hunter series)
Spring Dimensions
B-18/19 Factory spring specs Top Hat Spring Guide
10 ½ - 11 inches long Total length 1.250 Spring Guide diameter-.507
.117 wire diameter Guide Dia. .555
.527 ID Hat Dia. .750
.764 OD Hat Depth .190
35-36 coils
Coil Bound 4.224
B-20 factory spring
.500 ID Piston ID .745 Shaft Dia. .387 Depth 4.650
.738 OD Piston OD ..956
.119 wire Spring Guide ID .408
11 ¾ length Spring Guide OD .478
40 coils Chamber ID .983
Does this help ya my friend?

Courtesy of.... Taa Daa jist kiddn'