|For printed ckt board work, I think 0.035" is better than 0.050" solder. You want to minimize the heat used to flow the solder - to avoid delaminating a trace. Probably more important to use a low-wattage iron.
There is theoretical advantage to the eutectic alloy, 63-37, since it freezes at a single temperature rather than going through a pasty stage. But, for me, at least, the old 60-40 alloy works better. Maybe the brief pasty stage is more forgiving, at least for my ham-handed soldering skills.