Can anybody tell me what the real world advantage is to going with say a Garmin 305 Versus a Cat Eye wireless for a whole lot less? As far as I can tell for what I am looking to get out of it for immediate need the Cat Eye will work just fine, But is there a possible future use the Garmin might provide that I havn't considered yet?
The Garmin will allow you to record&analyze ride data, whereas the Cateye will not. You'd have to write down your ride details afterwards using the Cateye. Unless you are very diligent, this can be a pain. Also with a Heart Rate Monitor, a Garmin unit will let you graph your data so you can see not only where your high/low/averages are, but where on a ride you were doing certain efforts. If you do intervals, this is handy.
Speaking of graphing, a lot of cyclists 'coast' during their rides or generally ease up after a climb, probably too much, and if you can see (using a Garmin's data recording) where you are slowing down on a seemingly flat stretch, you'll start to realize that you can be doing more while on the bike. Think of how an indoor bike trainer requires you to pedal constantly, and how being out on the road, you get a lot of opportunities to coast along. This is a missed opportunity to keep your legs working. A cyclist who is doing it right can make a 30 mile ride the equivalent of another rider's 50 mile ride. You might see this discussed in Chris Carmichael's 'Time Crunched Cyclist' book.
I rode with a few different computers over the last 7 years, and finally got a 705 (also for the mapping features), and just being able to see and track my rides with a simple upload/connect to my PC is great. Garmin has a 'Training Center' software program that lets you look at your data easily. Over time, you can use this to work with various training programs, be it for a century ride or more serious racing. Plus there's the STRAVA site now, which you can upload ride details to and see how you stack up against others, if you're a little competitive.
If you haven't gotten any cycling training books yet, I'd recommend Lance Armstrong's book, or another one called 'The Ultimate Ride', by Chris Carmichael. He's also got one titled 'The Time Crunched Cyclist'. These are good for beginners or intermediate to advanced riders to help with cycling training programs. If you are more serious, I'd recommend Joe Friel's 'The Cyclists Training Bible'. Having one of these books will let you make maximum use of your Garmin.
If you're gonna use a Garmin primarily for cycling, I'd get an Edge 500. You don't really need the visual map features of a 705 or 800 model. I would recommend that you get a cadence sensor for it too, so that you can see how your pedal stroke is doing. This can be useful in relation to what I was saying above about coasting and also just making sure that you are using a proper ratio of cardio/muscle to ride. Too low of a cadence and you are probably killing your legs and not making good use of that blood pumping heart.