If you can examine the sweaters in person, figuring out which one is more to your liking is pretty easy.
If you can't examine them in person, well, I guess there are two ways to look at it -
Way #1 is "It's a crap shoot, since I'm buying sight unseen. I'd rather risk $70, than risk $150, so I'll buy the merino."
Way #2 is, "The lambswool is priced more than twice as high as the merino. Assuming even a loose connection between price and quality, that suggests the $150 sweater is better than the $70 sweater. I'll buy the lambswool."
And JBMarce21's point is valid, too. Even if the $150 sweater does turn out to be nicer than the $70 sweater, will it be nicer than two of the $70 sweaters?
Me? I prefer merino, simply because it tends to be a little thinner than lambswool, and is more comfortable to me. But that's just my preference; maybe you like a heavier sweater, or a warmer one. I'd add that I find virtually no sweater to be itchy, but if you are sensitive to itchiness, merino tends to be less itchy than lambswool. Again, these are just generalities - they don't mean that a specific merino wool sweater will appeal to me more than a specific lambswool sweater. Moreover, my preferences are based on the sort of sweaters I wear - almost always v-necks. I haven't worn a cardigan in years. So, again, your situation does differ from my own.
Still can't decide? Then buy the sweater offered by the merchant with the better return policy.
Finally, I'll mention that while Jos. A. Bank gets lots of criticism around here - and often deservedly so - I own a couple of JAB merino wool Signature v-neck sweaters, and am pleased with them. 'Course, I didn't pay the $100 "everyday" price, but instead picked them up for about $25 each, due to a sale and a coupon. So if anyone's thinking of buying some merino v-necks, and there's a 70% off sale on these, they're worth considering. Just saying, is all. (And no, they're not particularly trim fit. If that's what you're after, well, to each his own.)