propman I like Purdy but given the $$$ in your post, I'd opt for Fields. Reasons being you can pick up a number of quality players given the 45mm diff in annual payout. Convinced Shanahan could do wonders for Fields on field performance.
That's pretty much how I see it - for example, that level of difference in money is equivalent to Purdy having a skill position group of Aiyuk/Jennings/Piersall/whatever TE they draft or sign and hoping that their new LT can protect his blind side, or Fields throwing to Aiyuk/Samuel/Jennings/Kittle with Trent Williams keeping him clean.
That said, the Purdy extension won't be expensive in the first year - the starting point is the last year of Purdy's rookie contract - so San Francisco can probably keep their core offensive players together for one more year, but it starts to get messy in 2026, and quite a few of those key players are probably past their best by then (Williams will be pushing 40, Samuel may be on the downside already, and McCaffrey/Kittle are getting up in age for guys at their positions). There's a pretty strong case for San Francisco to follow the path that Buffalo went down this season, moving on from several expensive veterans who had been key contributors and bringing in younger players as replacements.