Fishfinder question

I am in the market for a fishfinder. I have been asking some opinions and reading some posts regarding different units. Hopefully what you guys tell me on this question will he me make up my mind. I plan to primarily crappie fish and do a little striper fishing on Norfork Lake (in north central Arkansas). I think I have decided on going with a Lowrance unit of some type, but I may go with a Hummingbird. Anyway, my question is - what is the reason for getting a 4000 watt unit versus a lower watt unit. Is it strictly dealing with the depth the unit can detect with precision? Does it affect the clarity of the display? I am having trouble deciding between a unit with GPS or one without, however, I was pretty sure I had narrowed my choices down to the X135 for a unit w/o GPS and the LMS-480M for a unit with GPS. Today I went to BassPro and looked at the different units and talked to the salesperson about the different units. While talking to the guy, he told me that if I was only fishing local lakes or lakes with a depth less than a few hundred feet that any unit over 1500 watts was overkill. He said unless I was fishing the Great Lakes or saltwater a unit like the ones I listed above were more that I would need. He recommended a unit like the X96 with 1500 watt power. What do you guys think? I don't mind spending the extra money if I need to, but on the other hand if it is something that will not make a difference with the places I fish, then I don't see a reason to spend the money. I appreciate any information you can give me. Thanks

I'll tell you, I've bought several units from 150.00 up to 300.00. I just ordered a 797c2 si because I want the side imaging.

Now for the cheaper units. I've had some of the Eagles, Garmins, Humminbirds, and 1 lowrance unit. The lowrance that I bought was a X96. It's hands down, the best low-end unit I have owned. I dialed the setting in within a half an hour on the water, I have arches for fish, and I see structure when it's there. I love the unit, and I'd buy another in an instant.

Glad to hear about the X96. I'm in the process of getting my 16' v-hull ready for the spring and have had my eye on it. Been seeing prices around $200. Thanks for your post, it's always good to find someone out there with a lot more experience that I've got and can confirm I'm on the right track