It depends on the player. Daric Barton is an example of a player who is exceptional defensively at 1B, and this adds a good amount to his value (assuming you believe that defensive metrics accurately value and measure defensive ability). His defense and ability to get on base (~.400OBA) are able to mask much of his relative lack of power as a 1B.

However, 1B is the least demanding defensive position on the field. Thus, it is the easiest to learn. Pretty much anyone who is reasonably athletic (and pretty much anyone who can play at the major league level) can be coached to be a mediocre 1B. An example is Adam Dunn, who may be the worst defensive OF in baseball (in 2009 IIRC, he played 90 games in the OF and was worth something like -37RAR). When he moved to 1B, he went to like -3RAR, still not fantastic, but only slightly below average and increased his value to his team by over 3 wins (1win~10runs). This is why I think Lind will be fine at first (he can’t be worse than Dunn) and picking someone up with a history of playing 1B should not be the Jays’ most pressing concern.

The point is that offense matters a whole lot more than defense at 1B. If you can get a guy who is exceptional defensively (like Barton, but Overbay doesn’t cut it), that can make up for some, though not a lot, of offensive shortcoming. But, if you have a chance to get a big bopper (like Manny), it’s easy to find someone to be decent at 1B (like Lind) and the offensive upgrade you get from the big bat will far outweigh the slight defensive downgrade from a good defensive 1B (e.g. Pena) to an average-to-mediocre defensive 1B (like Lind).

I think Manny will come at a similar or cheaper price than Pena on the FA market and that Manny will actually be a better hitter (maybe slightly less power, but way higher OBA). Given that the defensive downgrade from Pena to Lind at 1B will have only a small effect on the team’s overall ability, I’d rather have Manny’s bigger bat than Pena’s smaller bat and better glove at 1B.