Is one brand of unvented cylinder better?

Cylinder material - stainless steel will have a much longer life than copper or cast iron.

If the cylinder is fitted with an anti-corrosion anode, is it replaceable? An anode is not required for stainless cylinders because of the inherent very high resistance to corrosion. However, an anode is highly desirable on copper cylinders. On cheap copper cylinders, the anode may be non replaceable, which means the cylinders potentially have a short life as corrosion will begin once the anode is exhausted.

Does the cylinder take standard size immersion heaters? If your heater/thermostat fails, it's helpful if you can get a replacement. Again, some manufacturers use unusual size heaters, which may be difficult or impossible to obtain in later years, if the cylinder model is discontinued.

How much insulation is on the tank, and what is the standing heat loss (in kWh per day)? Are versions available with extra factory fitted insulation?

If you're using an indirect cylinder, what is the reheat rate?

Don't buy one at all. You need a BBA approved fitter and an annual service charge of £60-100 per ann.

Get a heat bank. The Pandora

Or look at a Rinnai high flow two bathroom multi-point. Far cheaper than an unvented cylinder. Very reliable - made in Japan. They even have a model that can be fitted outside.