A cruising catamaran, properly sailed is in very little danger of capsizing. Most cruising multihulls that capsize were due to human error.
I don't think the Iroquois is any tippier than the Gemini by any real amount, in fact, IIRC, the Iroquois has a higher beam-to-length ratio than the Gemini, so should be a bit more stable. But it really depends on which model of the Gemini and Iroquois you're talking about specifically.
However, the later models of the Geminis, the 105 series, are probably more stable than the previous models, since the hulls were redesigned to push the centers of buoyancy further out than the older hulls.
You might want to join the Yahoo Gemini Catamaran group and look at the Iroquois Catamaran Owners'
website.
I'm pretty sure that if either boat had a serious stability problem, that a lot more would have been written about it. Most of the stories I've read on capsizing multihulls, it was almost invariably one of two things. Either they were racing or they made a mistake.
In general, a multihull has to be sailed a bit differently than a monohull. A monohull will generally reef for the average wind strength and let the boat's heeling take care of the gusts. A multihull reefs for the gust strengths and doesn't worry so much about the average winds, since it can't heel to shed the excess wind. This is more true of catamarans, which have a much higher resistance to heeling than of trimarans, which can heel a bit.
Some of the dangers in monohulls, like broaching and rolling, are very much reduced in a multihull. Some of the IOR era boats, going downwind, can start to roll and then broach. A multihull doesn't really do this, since they tend to have much higher initial stability figures compared to monohulls. The fact that they have multiple narrow hulls also tends to prevent them from broaching. They also seem to do much better in "rolly" anchorages. I've anchored in a few spots and seen monohulls come and then leave due to the fact that they would start to roll a fair bit while anchored in the same cove as me, while I wasn't really affected by the waves.
Several good smaller cruising multihulls that you might be interested in are the Gemini, the Iroquois, the Catalac 8M, and the Heavenly Twins 26. All of these boats have made ocean crossings. The Catalac, Iroquois and Heavenly Twins that are in this country probably came over under their own power, since all three were European built IIRC.
It would help if you said what kind of budget you were expecting to have. I generally recommend that you reserve about 15-20% of your total budget for refitting, upgrading and modifying whatever boat you buy, since boats are not like cars and often need to be modified to suit the people sailing them.
I would ask what your sailing plans are, since that may affect what boat would be best suited for you.
You should be aware that a catamaran, while having a lot of stowage space, can't really be loaded down with stuff to fill all that space and still perform safely.
If you're interested in sailing on a multihull, I do have some friends that have a Gemini that sails out of the New Bedford region.
Keep it in mind if you and Mrs TB want another cruising boat, rather than a daysailer. 
