I'm a Mac 26 classic owner who's considering upgrading to a Seaward 26RK. Here's my perspective.
First, we`ve decided that trailerability is a non-negotiable feature, for us. Since you've already narrowed your choices to two trailerable boats I'll assume that's the same for you, and I won't bother to explain the many reasons I prefer a trailerable boat. Also, I`m assuming you`re thinking of a classic Mac 26, like mine, not one of the newer powersailors. Most of what I have to say about my Mac is probably true of the powersailors, too, but I have no experience with them.
What we love about our Mac is that it's very affordable. We paid CDN 9,500 (~USD 7,200) for a boat in like-new condition that does everything we need. It's easy to maintain, easy to tow, easy to launch and recover, sails decently, and has (just) enough accommodation for the two of us to be comfortable on multi-day trips. However, the accommodations are spartan. The "galley" is a swing out table with a propane camp stove strapped to it. The head is a porta potty (though enclosed). Water is a jug and a pump. So, basically, it's camping on the water. Don't get me wrong, we love it. But we plan to take longer trips in a few years, when I retire, and the Mac won't cut it for that kind of thing. Also, the Mac was definitely built to hit a low price point, with very ordinary hardware and minimal equipment. As an engineer, I appreciate that. If you can`t make money with your design then you`re not really doing engineering, and the Mac has served our purpose perfectly, up to this point, at very low cost. But, now that we`re more committed to the sailing life, we want something better.
So, our requirements for a future boat are that it must be nearly as easy to tow, launch, and recover as the Mac but it must also have standing headroom, a proper head with shower, something resembling a proper galley, and a reasonable anchor system. That's a very narrow field. I've considered quasi-trailerable boats such as the Nor`Sea 27. There are some great boats in that category (the Nor'Sea being, perhaps, the best), but they're probably not sufficiently trailerable for our needs. I`ve also looked at some of the more direct competitors to the Mac, such as the Hunter 260 or the water-ballast Catalina 250. Those are both good boats, but not enough of a step up from the Mac to be worth changing. So far I've only found three boats that meet all our requirements: the Seaward 26RK, the TES Magnum 28, and the TES 246 Versus. At the moment, the TESs haven`t been on the market long enough for used ones to be in our price range. So the Seaward is our leading contender.
I`m still early in the research phase but I haven`t yet found anything that would rule out the Seaward. It`s at the high end of our price range, but it ticks all the boxes. They`re reputed to have good handling qualities, and even pretty good speed for a 26-footer. Builder reputation is excellent. But there`s not a lot of information out there other than the company`s own marketing material, so I`m treating all that as tentative, for now.
So, it comes down to how much boat you want and what you`re willing to spend. If you`re prepared to lay out the money for a Seaward I don`t think you`ll find a better, truly trailerable boat. But if, like us, you`re a bit tentative about jumping in that deep right at the start, the Mac is a great choice to get you into the sailing life for not a lot of money.
(PS: I didn`t notice how old this post was before I replied! But I hope my comment will be useful to someone, anyway.)