The 2100 does make more sense sine the apu would be wasted in the 3850. However, the 2100 is just a dual core and games are starting to move over to quad core(I take it that you want to do gaming?). Could you stretch your budget to a i5 2300 perhaps?
Also, the socket of the 3850 has no future beyond the 3870 which is nearly identical to the 3850, while the socket of the 2100 will have upgrade paths.
core i3 2100 offers larger upgrade path, wider motherboard and chipset choices.
3850 offers strong igp performance, narrower motherboard and chipset choice. upgrade path seems to be locked so far. rumor is, amd will replace socket fm1 (3850's socket) with socket fm2 for future apus in 3850's class. then again, amd is known for backwards compatibility, so you never know.
this becomes easier with your gfx card choice. imo, the core i3 will be able to provide decent cpu performance with your gtx 550ti. if you can hold out till april/may you could even try getting ivy bridge after they launch and get reviewed.
But the A8-3650 rated higher than the I3 2100 in overall testing and remember if your going to overclock then the I3 2100/2120 are locked as well and no hyperthread capable its a tough call there both spec'd out well. and for the same price its a hard one to conclude. So depends on what your going to do with it I guess overall.
The i3-2100 is the better choice in the long run and it is pretty competitive with the A8-3850. Purchasing a P67 or Z68 motherboard for the i3-2100 means that you can upgrade later to an i5-2500k and overclock it. Depending on the motherboard, it could be possible to install an Ivy Bridge CPU (they will replace the current Sandy Bridge CPUs) with a BIOS update. Regardless, there is an upgrade window of about 1.5 - 2 years from now. When Intel releases the Haswell series CPUs mid-2013, all socket 1155 CPU (Ivy Bridge & Sandy Bridge) production will cease. Sandy Bridge production will cease when Ivy Bridge is released.
The only upgrade path for the A8-3850 is the A8-3870k. AMD's next generation APU, Trinity (this summer??), will use socket FM2 so upgrading to Trinity will require a new motherboard.