There are many things that make adhesives different from one another and right for one job and wrong for another, but there are a few main differences between HXTAL and XTR that may help you decide which is more suited for your particular job.

1. Cure time: HXTAL has a 7 day room temperature cure time. This allows it to be worked for a very long period of time and even allowed to be set aside to thicken up for more extreme jobs (such as glass to metal bonding). XTR has a 24 hour room temperature cure time. This is great for pieces that need to finish quickly, but it also means you have a far shorter working time with XTR (roughly 40 minutes from a 28g mixture). This limits the available uses for XTR.
2. Viscosity: HXTAL begins much thicker than XTR which allows it to cling into areas much more easily. It makes it vital for bonding pieces that have any gap filling to do between them. XTR is a much thinner epoxy and cannot be allowed to sit to thicken up (see point one above). This limits XTR's abilities to almost strictly flat glass to flat glass bonding and crack repair.
3. Environmental stability: Due to HXTAL's longer, slower chemical reaction it is less adversely affected by environmental changes such as temperature and humidity. It's like a slow steady train there for the long haul. With XTR, in order to get the faster cure time, but also maintain the clarity of the adhesive, it is much more chemically sensitive to outside variables such as temperature and humidity. Changes in humidity can cause very odd reactions in the adhesive and areas where XTR is used must be more tightly controlled than those with HXTAL use.