HP30 and the HP35 are more or less the same pant. They are the same as the HP520 pant with upgrades. There's a few minor differences such as the +1 zipper or zipper on the legs that one or the other might not have but fit wise they are the same and fit like traditional pants and are medium on the weight scale.
HP70 is very much a traditional pant and is a copy of the old Tackla 7000 / 9000 pants if you've ever tried those on. They fit like tanks, look like tanks, are as heavy as tanks.
HP45 and HPUCLP are the CCM RBZ/UCLP equivalent pant. They are the lightest pants on the market and provide adequate protection as well as seemingly(I don't have proof but there are very few other models used for CCM) being the most popular model CCM pant used in the NHL. UCLP is the older version and are a two piece pant (uppers and lowers can be separated). HP45 is a one piece pant usually with the +1 extension. The lining material on both the pants is slightly different and the lower back protector is also different, but all both pants function roughly the same.
Most pro stock enthusiasts that use CCM pants ride along with the trend and prefer the HP45/HPUCLP model over the HP35/30/70. The lightness of the pants with good protection(enough for NHL players to use) makes it the clear cut choice.
Sizing for the HP45/HPUCLP gets wonky because both pants fit a little short and small. I'm assuming "36-37" is your pants waist size and not your actual waist measurement. So, a large sizing guideline is 34-36 and XL is 36-38. Based on the small fitting of the HP45 and your waist size, I would go for an XL. However, since you're also 5'9 (me too but my waist is 31 and I fit a medium HP45 snugly), the length of the pants maybe too long. I find that being 5'9'' myself, the medium HP45 is just perfect with the +1 extension so XL for you may be too long.