Rechargeable? The Garmin Forerunner 101 is the only one that is not rechargeable, it uses 2 AAA batteries. All others have a rechargeable lithium ion battery that lasts between 10-15 hours depending on usage.

Waterproof? All the Forerunner units have an IEC 60529 IPX7 rating, which means the GPS case can withstand accidental immersion in one meter of water for up to 30 minutes. Notice it says ACCIDENTAL....do not use these devices for the swimming leg in triathlons, they will not hold up for long!! Running or biking in the rain will not cause issues.

Computer downloadable logs? Again, the 101 is the only device which does not have a computer interface, so the information that is kept must be deleted at some point. All the devices are capable of holding approximately 5000 laps, which equates to around 2 years of data.

Size & Weight? The 101, 201, & 301 are virtually identical in size and weight. The 101 is slightly larger at 3.3"W x 1.7"H x .9"D compared to the 201 & 301 at 3.3"W x 1.7"H x .7"D. Believe it or not these are extremely lightweight. The 101 weighs in at 2.6 ounces with batteries, the 201 and 301 both weigh in at 2.75 ounces. The 205 and 305 have a different design which looks more like a watch. They both have the same measurements at: 2.1"W x 2.7"H x .7"D and 2.7 ounces in weight. Both designs are very comfortable and lightweight. My opinion....there is not enough difference in weight or designs to make a decision from.

GPS Signal Strength The Forerunner 101, 201, and 301 all have the same antenna and the 205 and 305 share a antenna design. Between the 2 groups, the 205 and 305 have the stronger signal strength with an improved antenna design which points at the sky while running. The 101, 201, and 301 all will occasionally lose signal while running under dense tree cover or around large buildings. The 205 and 305 don't seem to have this behavior.

Heart Rate Monitor: Only the Forerunner 301 and 305 have a heart rate monitor option. This allows you to display and track your heart rate and configure audible alarms if your heart rate goes above or below certain thresholds.

Cool things: All 5 devices have AutoLap, AutoPause, Virtual Partner, Pace Alerts, Time or Distance Alerts, and Interval training options. AutoLap creates a lap for you at an interval you define (1 mile, 1 minute, 30 minutes, etc...). Auto pause will pause the timer if your pace falls below a threshold you specify. This is a nice feature if you have to cross a lot of streets and are waiting a lot. The Virtual Partner feature is for those of us competitive types who have to have the sense you are beating something!! You set up a predefined time/pace and it shows this 2 little people on the screen, one is you, one is the computer. It then tells you who is ahead of who and by how much! Talk about motivation....can't let a stinking computer beat you!!!

Specific features for the Forerunner 301 and 305: As already mentioned, the 301 and 305 have a heart rate monitor capabilities. They also have the ability to have multi-sport profiles (biking, running, one other sport) and advanced workout features. The advanced workout features allow you to set up workouts with specific durations and targets (speed or heart rate) so you can follow them. Things like 4 1 mile intervals at a 8:30 pace with a 1 minute rest period between intervals. It then beeps to alert you on your intervals.

Do they tell time? Believe it or not, this is one of the most asked questions and yes, you can configure one of the screens with many different pieces of data. Time of Day is one of them.

Foreign Language and Units of measure? All units allow to change the language on the console and the unit of measure to meters if so desired. The manuals only come in English however.

The Forerunner 305 also has an optional GSC 10 speed cadence sensor you can purchase for bicycling.

All 5 of the Forerunners are extremely valuable in your training and you can't go wrong, no matter which one you choose.