I would say go with the Boston acoustics for a few reasons:
a) you'd have a matched set b) most people will agree that components (the oens with the seperate tweeter) are better in general. c) you sound as if you prefer them to the Alpines already.
Your description of the Alpines sounds like my Infinities. they have fantastic bass response, but the mids are lackluster. The highs were terribly harsh on them too, but fortunately, the tweeter design of the Infinity Reference series is a bit unique (the front ones can be rotated to different angles, and the rears can be mounted so they're either 30 degrees forward or 30 degrees backward) , and I was able to play around with and tweak them to my liking. Honestly, I don't think I'd buy the Infinities again, although I did get a fantatic price on them.
One thing - if you get the BA components, it is recommended that you disconnect the factory tweeters in your upper dash and have the Boston Acoustic tweeters mounted in a better location. A lot of people use the triangular panel that is part of the sideview mirror, and some people put them lower down somewhere on the doorframe. Since I personally didn't get front components, I can't comment on that.
If you feel like you're lacking bass, you could always try investing in a sub or perhaps something like the Infinity Basslink II. A few people here have that one and say it's great.
