My Enco is the same as the Eagle 30. It has an R8 taper. It looks like you can get the Eagle 25 in an R8 as well from that Chester site -- see the dropdown list.
Of course you may not want an R8 -- but I like it -- it was the standard for Bridgeports. I do think the ER collets are what most want, and I will probably go the adapter route myself some day as well. But for now the R8 works very well for me.
It has the disadvantage that my lathe accepts MT3 so they aren't directly compatible, but of course with adapters or collet chucks you can solve that.
It's impossible to say which of your choices is better, at least for me. The above mentioned advantage of a fixed slide instead of a round one is real. However, I would not personally trade my machine for the square slide one you mention. I believe jn weight/mass and mine is double the weight of the other. The HP rating is higher, I believe and the carriage travels are longer.
But, not having owned the square type, I can only give a suspicion, and that is that the square type might be built more precisely/better. Don't know it for a fact. Nevertheless, I still wouldn't trade, because I know that whatever I don't like about my mill (and there is little so far) I can change, and it is a more solid base, with more power and capacity.
If I were doing small models instead of some of the rough stuff I do, maybe I would choose the other.
One thing you should definitely do is visit the distributor and actually look at the real mills, turn the cranks, etc, and get an impression off of the computer, in the flesh.....er metal.
And don't ignore the possibility of an older used mill -- possibly a British or American mill in your price range.
