The Boss GX-700 is similar to the Roland GP-100, but costs less. In order to achieve these cost savings, Roland trimmed a few features from the GX-700 which are present in the GP-100. The GX-700 came out a year later than the GP-100 though, and Roland took advantage of this time lag to improve some usability features. The following discusses some of the differences between the two. I didn't bother to mention features here which are common to both, so if you're looking for a comprehensive list of features for either one, check out the Roland web site.
The GX-700 does have a couple of extra effect controls missing on the GP-100, though. For example, the flanger has a gate and the reverb has a diffusion control.
The GX-700 has a couple of effects, humanizer for making vowel sounds and a ring modulator, which the GP-100 lacks. However, the GP-100 has a slow gear for violin-type attacks and a feedbacker for infinite sustain which the GX-700 lacks. All four of these effects are of questionable utility, though. I don't know why Roland included those, but left out a rotating speaker effect.
The GP-100 has more "pro" features like stereo effects loop (GX-700 is mono), two pairs of outs, 1/4" headphone jack instead of mini, switchable line or mic level input, a configurable set of 4 preamp settings which can be applied to any patch, and global eq that is applied after the effects programmed into each patch
The GP-100 presents an amp-like user interface by offering a set of knobs much like amp front panel controls. The GX-700 is laid out more like a pedal board with a set of buttons, each of which corresponds to an effect. This makes it easier to jump right to a specific effect so you can tweak it. On the GP-100 you have to scroll through the effects to the one you want before you can edit it (although the knob allows you to skip through the effects quickly). The names of the effects are printed on the front of the GX-700 though, so if you change the order of the effects the labels on the buttons are no longer correct.
The GX-700 permits 13 simultaneous effects. One of these is "modulation", which includes one of flanger, phaser, pitch shifter, harmonist, vibrato, ring modulator, or humanizer. This means you can't use more than one of these at a time. The GP-100 instead offers 5 different algorithms, each of which offers a different combination of up to 16 effects. So, by choosing the Delay algorithm you can simultaneously have a phaser, a flanger, and a harmonist. On the other hand, there's no algorithm which has both a tremolo and a flanger.
Speaking of algorithms, the GP-100 has a neat feature absent in the GX-700 which is called Dual. Basically it permits you to use two patches simultaneously. With this you can get a sound which is the combination of two different amps. The GP-100 has a ducking delay, which silences the delay while there is input from the guitar. This is an invaluable feature for preventing delays from sounding muddy. The GP-100 harmonizer allows you to delay the voices, so you can get arpeggio effects.
The GX-700 allows you to simultaneously have a distortion pedal and an amp simulation at the same time, whereas the GP-100 only allows you to have one or the other.
The GX-700 has a couple of nice usability advantages over the GP-100: it allows you to compare the editted and uneditted versions of a patch and has an edit undo. Also, the front panel lights are used during tuning, in case you're so far away that you can't see the display.
Also, I thought I'd prefer the GX-700's user interface where an effect is always one button press away. But, for me I found it inconvenient to have to press the button, then scroll using the parameter buttons to find the parameter of interest. On the GP-100 I can press-turn the parameter knob to select the effect, then continue turning the _same_ knob to find the parameter of interest. I guess I just find it more convenient to use one knob to select a parameter rather than two buttons. When I'm reaching to a rack-mounted effect it seems to be more efficient to leave my hand on one knob rather than switching between buttons.
Having the GP-100's front panel knobs was the clincher though. It's so nice to just reach for a knob and turn it to crank the gain or adjust the tone.