i've used KONI Sport (yellow, adjustable) on 3 cars now, including a '94 MR2, my old 156, and now the GT. i've always been very happy with the results: much improve damper control for less 'crash' into potholes and over ridges, which improves ride quality overall. the 156 and GT had/have Eibach Pro Kit springs fitted.
i'd suggest doing both front and rear at the same time. obviously this doubles the cost, but having only the fronts done will not provide the ideal front-rear damping bias. handling won't be optimal until the rears are done too. $450 sounds about right, having paid somewhere between $800 and $900 for front/rears together.
i like the idea of adjustability. i tinkered a fair bit with the 156, especially after adding a very heavy stereo boot-installation; it was a boon being able to dial in extra damper resistance on the rebound. and overall, it can alter turn-in characteristics if the rear is firmed up relative to the front (note: turn-in will probably suffer if you only install front KONIs if the factory rears are relatively 'soft'). i've tinkered only a little with the GT. seems that anywhere from 'full soft' to about 1.5 turns offers a good compromise (they are capable of around 2.5 full turns of the adjustment knob).
mine have been installed by my local alfa dealer (Lance Dixon, Doncaster) rather than the KONI retailer. i figure alfa are the best for that job, having the previous experience and all the right tools.
i've not used bilstein, so can't compare. only read that they are also excellent.