Not sure what I did, but my first reply disappeared, darn it...
I haven't dealt w/ Double Diamond. BUT I have purchased a Tough1 nylon harness and found it to be not worth what I paid for it (my home made, braided from re-cycled haystring w/ SS hardware is better quality!). Even before I had a qood quality harness (I've had quality, leather, QuickHitch Hackney training harness for years, tho), I knew that the hip strap assembly and crupper didn't fit right and weren't really adjustable for my mini sized Shetland mare. The headstall/bridle is too large - nylon too wide but it's too small to fit a larger pony that the wider nylon would work with. The hardware is substandard - rusting & oxidizing before I'd even owned it for 90 days. Then when grounddriving the mare before hooking her (she was highly upset), the left side rein & the check rein turrets both broke (really glad I was safely at home & that she wasn't hooked!). Later, came to the conclusion she was upset by the ill fiting saddle when I switched it out for one that I purchased at a used tack sale. The Tough1 harness went into the trash. The other saddle was matched by an Amish Harness maker in OH (the used tack sale only had the saddle w/ shaft carriers & wrap strap, the hip strap assembly and the crupper) - w/ a headstall that fits (open but with the availablilty of purchasing the right size check straps w/ blinders) & a breast collar and traces. It's not ultra fancy - but it is well made & worth the $100 that I spent on the parts (I don't have matching lines yet - kept the ones from the Tough1 bridle).
On the Bio one from Double Diamond, made by Amish harness maker, you might be better to go directly thru an Amish harness maker. I did another post that listed several in Millersburgh, OH - Bowman's Harness 330.698.3200, Valley Harness and Fairview Country Sales 330.359.1501. All three will take orders (might be hard to get up with them - but it's not impossible!) and will work with you on custom fitting for the same price as what they list in their catalogs. All have harness in the pricerange you list - and if you start at the lower end of the price range (both Valley and Fairview has an economy harness) - you can upgrade later by piece or part... Valley is working on doing a shaped breast collar for the VSE's like the one that Chimacum sells. I did like the leather one in full horse size that I was shown. I think that Bowman's also does a shaped collar - not sure that it comes in mini sizes though! I have Shetland and welsh driving online freinds that have had their harness(s) from these makers for years and they are very happy with it. You can choose the type of hardware you want too - brass, SS or plated (yuck). Fairview's economy harness is the least exspensive - with more rivets than the other companies... But still not bad. I haven't had the chance to USE a saddle built by any of these 3 - but did like how they looked when I visited there 2 weekends ago. I think all three of these companies will make a higher quality saddle w/ sliding ... shaft carriers (can't think of what that option is called). Yes, it will cost more but...
I can't remember what the first harness you show in your post is.
I prefer biothane over leather as I have MANY ponies & I no longer enjoy the time it takes to care for leather, it's hot & humid all the time here, I don't have a properly enclosed tack room and my barn drips on everything stored in it. The bio can be tossed over the stocks and washed or even washed on the sweaty pony! I love that feature. I've seen better quality bio made by
Country Carriages USA right here in NC that looks so close to leather that it takes a 2nd, hard look to determine that it's not - but it's currently out of my price range at more than $500 for a basic single, small pony harness. Two mares that I sold were campaigned in Southern Pines in the deluxe harness as a pair and as part of a 4 up by Deb Branson.