I have both the Mini and the Eco. The Eco feels lighter in weight, just slightly, than the Mini though it is longer, yet it doesn't feel cheap. If you like the feel of the section on the Mini, and the transition in size from barrel to section, then you'll be fine with the Eco.
My mini has a Fine nib that was re-ground and is about the size of the extra-fine on the Eco. Even though my Mini nib is a Pendelton Brown italic, it feels no smoother than the factory-standard extra-fine on the Eco.
I'm liking my Eco a lot. It's a great everyday pen, holds a lot of ink, looks good and I like the single-piece body (barrel and section are all one piece of plastic) as it's one less joint to spring a leak. I am using Sheaffer Burgundy ink (vintage) in it right now and it's not too dry or wet, but that's a well-behaved ink. It's laying down a typical extra-fine line.
And as for swapping nibs, just going by Goulet, it will cost you about $6 more to just get a whole other pen than to buy a replacement nib unit. And you don't have to worry about trying to pull the metal nib and all of that hassle. Sounds like my 30 minutes (or more) of fiddling with the replacement nib is worth $6.
Every day I use the Eco I'm more and more astounded by the quality for the price. It is definitely giving the Lamy Safari a run for its money in a slightly less eccentric-looking package. (don't get me wrong, I like the Safari and Al-Star, but it isn't a traditional look, which is what some people like about it) Right now, the main advantage the Safari has over the Eco is the availability of cheaper replacement nibs. If TWSBI starts selling just the metal nib for the Eco for you to swap it out, at a lower price than the current nib units, you've got a real battle on your hands for best beginner pen. (that's not a Chinese or Metropolitan)
I'm definitely going to get another one once the next wave of stock gets in, and this time it will be a 1.1. And then the next one will be a fine.
Frankly, I suspect the Eco will cannibalize sales from TWSBI more than Lamy in the short-term. I was wanting to get another Mini, or a Vac 700, but I can get a couple of Eco's for one Vac700 and have change left.
My recommendation is to get one. It's well worth the modest sum.