Not ever owning both products I will not comment as to which is better. I can only tell you about my experiences with each during my decision making process and with my experience with the one I purchased.
After reading and browsing each web site several times I decided to contact each and get estimates. On the Mianne site I was able to browse their catelogue of "stock" plans. There I found a "U" shaped layout that, with some modifications, would make the best use of the room I had for my Train Room (plan #11). I called Tim Foley at Mianne one afternoon about 4:00 pm EST. I asked if he would be willing to give me a price for my bench work. He asked if I had a plan, I told him I was interested in #11 with some modifications. While on the phone with him I re-measured the entire room and gave those measurements to Tim. He asked if I could give him about an hour and he'd get back to me with two plans, one at 30" and one at 36" wide with 48" square corners. About an hour later he called with the prices for both including shipping, then he e-mailed me the plans. After discussing the specs of his product and the reasons for the way he builds it and assuring me that there's no presswood or particle board involved, I told him I'd get back to him one way or another after I got a price from Sievers.
I called Sievers and asked the same series of questions and asked if I sent them a scale diagram of what I wanted could theygive me prices. Although extremely friendly and polite, I was told I'd have to make copies of their template pieces from their web site, afix them to my drawing and I could either mail or fax it to them. Once I did that they could tell me the price, so off I went to copy, cut and glue these squares and triangles to the drawing and faxed it to them a couple of hours later. The next day I received a fax from them with the price and list of materials needed but the shipping was an estimate and I'd either get a refund or another bill once it was actually shipped.
Both products appear to be quality items. I'm not going to debate the pine in the Siever's product versus the poplar and MDF used in the Mianne product, the Mianne product was about $200 less. So I went with Mianne for price and the all important first impression made by Tim Foley.
Now on to my experience with the Mianne product itself. I discussed a ship date with Tim that worked for both of us and on that day the boxes were shipped, 3 of them totaling about 100 lbs. and arrived the next day by UPS Ground (Boston area to Putnam County, NY). They sat in the garage for 3 days before I decided on a Friday to "take inventory" of the parts, just in case any were missing or I was not happy with them. Well, about 3 1/2 hrs later, all but 10 pieces of my 10' x 10.5' x 12' x30" (48") bench work was up. Assembly was easier than expected, the general assembly instructions are detailed and well written and included with the parts list for "your" bench work is a color coded diagram and instructions sheet of your bench work for you to follow.
Sorry for the long story but I'm please with my decision to use Mianne. The structure is relatively light but strong. That's my 2 cents! I'll try to post photos of it later.