Some major differences between 5205 and 5105:
(a.) 5205 uses TX-SR307 avr whereas and about 10watts underpowered compared to 5105 that uses the higher series avr (570 I guess) and is similar to TX-SR506 (but 5.1ch).
(b.) 5205 has 10" downfiring active woofer 150watts and 5105 has 8" downfiring subwoofer with 90watts. The sub on 5205 is more smooth.
(c.) 5205 have the following inputs: 1 x coaxial, 2 x optical (toslink), 3 x HDMI (1 for output - passthrough only, not upscalable), Speaker A x banana jack input
(d.) 5105 have 2 x coaxial, 2 x optical, 5.1ch inputs, 5 sets of banana jack inputs.....
(e.) 5205 have smaller sized satellite speaker and larger subwoofer cabinets. 5105 have larger front speakers, and smaller surrounds and subwoofer cabinets.
(f.) 5205 have glossy speaker cabinet finishes that will scratch immediately upon unpacking.......I used Meguires's Scratch X and No. 26 + 21 to remove the scratches (followed the same procedure with my car).
5105 has a better avr. Try looking for the outgoing TX-SR506 at a bargain price from your dealer, and add separate speakers (Wharfedale Diamond 9 matches well with Onkyo avr).