As above there was no diesel in the MK1 IS, the Sportcross was available as a 300 only to begin with, and then the 200 was made available in 2003 I think.
The IS is a competent cruiser, but I'm pretty sure nowhere near the comfort levels of the LS - I personally think the 300 will be the better choice for you, there won't be much in it in fuel and the extra power and refinement will be worth more to you.
Best to take one for a test drive and see how you go :)