I add my vote to 'BG1 first':
The mystery to be solved in BG1 will be spoiled in BG2, immediately from the start: you'll rob yourself of the experience that can only be experienced once, of trying to figure out why it is assassins are after you in BG1. Don't play BG2 beforehand, don't read entire walkthroughs and avoid spoilers on fora. You only have that opportunitiy for wonder once, don't spoil it.
Higher level characters are harder to kill, but you'll get to know the magic system much more intimate if you familiarize yourself with it's spells one level at a time. To survive the first few levels in BG1, it will help you if you get to know what Thac0 (to hit armour class 0), what AC means (AC), and why you should strive to get those figures low, learning the value of the sleep spell, learning to run away and pepper the enemy with arrows. To defeat enemies in BG2, you'll have to get to know a lot more spells, what they do, what they counter.
Final argument: there's a lot of satifisfaction to be had in climbing in experience from someone who comes just out of the study hall, wet behind the ears, to a mighty warrior or mage (cleric or thief) who can take on enemies at the end of BG1, you could only give a wide berth at the beginning.