Get the beater. Think of it this way. Every mile you DON'T drive the IS saves you about 10 cents of depreciation. Depending on your driving habits, if you drive the beater for 5,000 miles during the winter, you're retaining an extra $500 in value for the Lexus.
It's not the cost of the beater vs. tires you should be comparing. When I had my IS during the winter, it was completely filthy and I never got a chance to clean because it was too damn cold. Chips from sand/salt throwers, nasty potholes, not the weather I want to drive in with my IS. With my POS beater, I would care less what happens to it, it's a beater.
Plus my insurance went down. Now that my beater is primary and the IS is secondary the total insurance payment is less than having just the IS. This may or may not apply to you.
ya $800 AND it has brand new all weather tires on it, i think my only risk is the engine blowing up cuz as long as it drives IM GOOD. As far as comments about getting a better beater, the point here is to save money AND keep the IS in one piece. The bonus here would be not HAVING to drive it in the snow and salt.
I had a probe GT before the IS and had a variety of Street Performance tires on it, the last being Nitto 450's. It is not what I would call a good car in the snow. Part of the problem was the tires....which or course could be fixed at a cost. The other reason it was poor was because of the poor clearance it had in the front. This may only affect the GT model, but the stock body had a low front end that would plow through snow if it was high enough.
Since you're talking $$, keep in mind that you can sell the probe at the end of the winter also. I traded a proble for the IS and if I still lived in a snow belt state, I would have kept it in a heartbeat, given what I got for it when i sold it.