Silverstreek is in the right way of thinking here. Generally speaking, when it comes to electronics, buy the most up to date models you can get. They come at a price premium, but will remain relevant, supportable, and will work with other upcoming electronic equipment for the longest possible time before they're subject to planned obsolescence.
When it comes to radars, buy the largest type of unit that your budget and installation will support. Radar performance is directly tied to beamwidth and power, there's not really any way around that. While new radar technology has vastly improved the performance of the smaller domes, the fact remains that bigger will always be better, and a larger dome using the same technology will outperform its smaller counterpart. I myself violated this rule of mine on my electronics upgrade, and am now trying to figure out how I can upgrade my Halo 20+ to a 24 without taking a huge hit. Not that I'm displeased with the 20+ performance, but... bigger is always better.
Regarding the very real cost difference between units, a clever way that I've found to offset that if you're not in a hurry to buy is to leverage West Marine's reward points system. A few times a year, if you're a Gold Member, they offer a 5x Points event. This also applies to online purchases, as you're very unlikely to find anything above entry level electronics in their stores. While WM gouges on some items, the price for electronics is generally set by the manufacturer so there will be little difference between WM and other vendors, but you can earn pretty significant rewards points to the tune of a few hundred dollars on major items. For example, on a purchase of two MFDs, a Halo radar, and some miscellaneous items, I got enough rewards points to purchase a NAC-2 autopilot computer. I also took advantage of shipping them to a store (and thereby avoiding shipping costs) in a location with a significantly better sales tax structure on those items
All in all, I basically got an AP setup for free, minus the pump which I already had.