I'm in the midst of a re-power now, I choose a Beta 38 to replace a 30 year old Westerbeke 50 (41hp). The primary rational for choosing the Beta was it would be the best fit with little or no engine bed modifications needed. I also found that Beta has considered access to regular maintenance items important and that has resulted in some logical engineering. The impeller, oil filter, oil pump, etc are located at the front of the engine, a dream compared to my old Westerbeke 50. I will actually enjoy changing the filter and impeller and be able to do it in a matter of minutes vs. hours, that = a better maintained engine IMO.
Most T37C are or are approaching 30 yrs old, several owners have already completed a re-power. I found a few who went with a Yanmar (3JHE), another with a Westerbeke (44 I think?) and a couple with Beta. Both the Yanmar and the Westerbeke required significant modifications to the engine bed I believe. The Beta allows for custom feet (added cost of course), and therefore has resulted in an almost perfect fit, only requiring the drilling of new holes for the forward bushings/isolators. Thats is pretty awesome and worth a lot in my opinion.
Yanmar is one of the only purpose built marine engines I know of, maybe Volvo as well? Some feel that is important, I don't. I know Beta had some issues early on in its development but has made some significant improvements to their marinization of their line of Kubota based engines (IIRC, it was a poorly engineered heat exchanger, since redesigned). Nanni, another line of Kubota based engines is very similar to the Beta line, worth doing a comparison.
By the time the fall boat show came around, I had narrowed my choice down to a Nanni or Beta, ultimately Beta won my business because of the response I received. After my initial contact with Beta Marine US via email, I received a call the following day. Since that initial call, Stanley at Beta has been quick to reply and answer all my questions satisfactorily and then some. I cant say that was the case when I contacted Nanni, Yanmar, and Westerbeke. Excellent customer service goes very far in my book, if the engine is a good one that is yet to be determined...I'll let you know in about 30 or 40 years, I hope
I think you will find they are all pretty closely priced, and they all have different selling points that you may or may not find valuable. A Goggle search using tags such as Marine, Diesel, Problems, etc should yield you some results worth reading. Also, most of the engine manufacturers have specific forums, I know Beta does. Some time spent searching them and see what the threads read should also provide some insight.
Often you will find folks will tell you that their choice was the best because they chose it (I am trying not to do that here). Are they really giving you unbiased information, or just their opinion and want to tell you what they choose was the best because they chose it?