I've tried both, and there are aspects of both that I like and dislike, much like Ned. For now, I'll keep my main stash listing in a LibreOffice database, but will consider duplicating it on MHI or SM for backup purposes.
Scalemates:
What I like--I uploaded nearly all of my stash into Scalemates because I found its search engine fairly easy to use and its database very extensive--it was only missing one of my kits. I usually search by product number, and I had no problems finding the exact kit--often it would be either the only hit or one of a handful of hits. I also love the very large AM database Scalemates has--it's handy for when you need to find out if there are related products. I've found the actual adding of items to the stash almost as easy as with MHI (but not quite)--you click on the green bar next to a search hit, click on stash, and click on add. You can then search again, right from the same window--this makes the process of adding to the stash very quick. I also like the ability to create projects that include a kit (or more!) and several AM items. Finally, I like the ability to add new kits and AM items--this allows the database to grow quickly (and you can correct items that aren't quite right).
What I dislike--I've also uploaded some of my AM products, but I don't like that I can't assign AM products to a specific kit; they just get dumped into your stash. SM tried to group them with the kit they belong to, but some AM items are universal, so they just get dumped into a category (often a counter-intuitive one, or one different from the one listing the kit they are intended for). I would like to be able to link AM items to a kit, like in MHI. I also dislike the inability to change the stash categories in SM or to create additional stash lists. SM decides for you where a kit goes, and some of those decisions are not exactly kosher (a Mini Cooper kit listed in the same category as a Bundeswehr Wolf kit and the RB Model US Antenna bases set?). You also can't assign prices paid or dates of purchase to specific kits. The one thing about its search engine I dislike is that it tries to give you suggestions, and as soon as its starts trying to do this, you can't hit Enter to start the search (at least on my browser)--this is irritating, as I either type fast, or have to use the mouse.
My Hobby Info
What I like--Its ability to create different stash lists is a big one. I also love the ability to assign AM items to a kit. Adding kits once you have run a search is very easy--just click a button. The ability to narrow a search is useful, as is the ability to search your stash for a given kit. Listing a price and date acquired is a cool and useful feature.
What I dislike--I'm not enthused by the kit selection; it's OK, but it has limitations. I also don't like that to add kits you have to request that they be added--you provide the same info on the kit that would end up in the listing in SM, in any case, so it would be more efficient (and the database would grow faster) if users could add kits on their own. I am not keen on how MHI handles completed kits--they go into a separate list (that's good), but they are not deleted from your stash or marked as completed in the main stash listing (that's not good). I would prefer that they either be removed from the stash listing and placed in the completed kits list, or marked as "completed" in the stash. I would like to see MHI add AM items to the database--this would be a big plus.
Minor niggles--While not a major issue, one added boost in functionality would be the ability to assign AM items to more than one kit (decal sets, for example, can often be used on several kits). Another one would be the ability to assign a status to items--"building," "part built," "for trade," etc.
