Like MrTom said, Harvest Moon is a much more structured game and has a more traditional focus on progression and levelling up various things. The HM characters are much duller than the AC ones and their conversations quickly become monotonous.
That said, most HM games have an absolute ton of things to do in them and unlocking everything 100% and collecting/discovering everything may take 100+ hours. However, it very much depends which game you choose - there are good HMs and less good ones. The latest one, HM A New Beginning was released a few months ago in the US and is by all accounts very good, if you're a fan of the series. Who knows if/when it will come to the UK.
Tale of Two Towns is also very good and you can buy it easily in the UK. I imported a regular DS version a good while back rather than wait for the 3DS version.
The important thing about HM is that you have to be interested in farming/growing stuff (I have a veg patch in the back garden) and you have to not mind somewhat repetetive game play...till soil, sow seeds, water seeds, harvest plants, feed animals, etc. Much of the pleasure of HM is in the discovery of new or hidden things, a fair few of which will take quite some work to uncover. It's charming.
You will eventually reach an 'end game' state in HM where you have done most of the easy stuff and maxed out everything and have a profitable and productive farm. This may take 3 or 4 in game years, or even less if you grind and only grow the most profitable crops etc. So, HM may not have the long term legs that you're looking for. For me, it's a classic series and I eagerly await each new title in the series.
On the other hand, AC doesn't reach this end game peak quite so quickly. All told, AC has at least 1 year of daily gameplay in it (and who knows how the new improvements will extend that). AC is also a game that is much better when more than 1 person plays it. You then have a whole meta game thing going on there. HM is really single player only. AC is also more cheerful and creative, and offers the player a lot more freedom about what they want to do. HM is more like work, and AC is more like play. But in the end I guess they can both be whatever you want them to be.