I'm certainly no expert, but I can offer my observations and input from my local shop and White Bros., who've done a lot of 440 kits. I have two XR4s, one a 96 with a 440 kit, which I bought used a year ago.
The 440 kit is is considered by the shops to be a very reliable upgrade. Performance-wise, I notice a significant improvement in low-end power, which I like for my kind of mountain trail-riding. I don't notice a lot of difference in top end, which you may desire for desert riding.
I've compared performance with a 01 XR4 and a 98 with the HRC cam. The cam'd bike has similar performance mid-top end, but not as good on the bottom. All comparision bikes are set up similarly with (pipe, airbox, etc.), although age/engine wear may also be a factor, since the 01 XR4 is not far behind.
Also, I noticed a few months ago riding in the desert, my head gasket started "seeping". After checking with the shops, this apparently is not unusual since they claim the aftermarket head gaskets aren't as good as stock. I also suspect I was running lean and hot, because it stopped with cooler mountain riding.
BTW, The shops recommend a cam upgrade with the kit. I still have the stock cam. One shop doesn't like the White Bros. cam, except for racing, because they claim those cams are made from modified stock "cores" and have rapid cam lobe wear, compared to the HRC cam.
My XR4 riding group's amateur conclusion is that the 440 kit seems preferable for trial riding (better low-end) and the HRC cam seems preferable for desert. Or you can go both. Also, if you have a lot of hours and remove the head, valve seal replacement might be a good idea, since we notice these fail frequently due to high head temps.
I'm not sure the HRC cam is still available and it's pricey. But, if I rode mostly hot flat-out desert, had a newer XR and wanted the best reliability, I'd go with the HRC cam.