I would think about it like this (since I know nothing about your team or league):
Yes, there are a lot of 1B that put up big numbers, but if you keep Hosmer you're likely to have one for the next 10 years. Its one less thing to worry about. Even though Hosmer is going through a major sophomore slump right now, remember he's only 22 years old. Hosmer has potential for .300/30/100 years in the future. That's top 5 position, probably top 20-25 overall...pretty solid.
There are A LOT more outfielders in general than 1B, which means there are a lot more guys out there that could match Jennings's numbers long term. Jennings is more of a .260/20/65 and 20-30 SBs a year. There are a lot of outfielders out there that can come close to those numbers (maybe not the SBs, but the average and power are mediocre at best). Jennings is 25 years old, so you'll likely get less time out of him. Plus with speed being his best tool, that is usually the first thing to go with age. You'll likely get a max of 5 years at peak production. Think Carl Crawford (Crawford is 30 and is already on the down-slope). Jennings probably has .300/30/100/35sb, and might get you 1 or 2 years of that, but it won't last.
I personally would rather have a potential top 20 guy who will most likely be more stable long-term (he reminds me A LOT of a young Joey Votto). Yes there are proportionally more good 1B than OF, but knowing you have a solid one makes it easier to draft a team around him, and let everyone else fight over the Votto's and Cabreras of the world.
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