United and Marriott have teamed up to recognize each others loyalty program members and status holders.
As United Premier Gold, Premier Platinum, Premier 1K and Global Services you will get Marriott Gold Status through a status match. Marriott Gold status isn't bad bad given the many exclusions in Marriott's Terms and Conditions it's a big lottery what you'll get besides free Internet. If you have status definitely go for the match!
I can't see any reason why I would transfer United miles to Marriott Rewards points at a 1:1 ratio so don't do that.
For Marriott elite members they can match themselves into United Silver. This is nice to have but given the low priority of upgrades to domestic first not much of a benefit. It does NOT offer access to the United Club or Star Alliance Gold.
You are able to transfer Marriott Rewards points with a ratio of 5:1 to United miles. Again that's not worth it.
As you may know Marriott offers a Status Challenge for Gold or Platinum called 'Taste of Gold' or 'Taste of Platinum'. You can get such a challenge every 12 month for your Marriott Rewards account. It will upgrade you right away and require a varying number of stays/nights to make permanent for the year.
If you need status with Marriott or United status this may be a good way to get started.