First, the all-important numbers:
|Comparative sizes shown are in proportion to each other|
The radios are quite similar in what they do and in the advertised specs so the differences will primarily amount to how they perform various functions.
The rigs have a different band selection process with the HB-1B accomplishing this via built-in memories (of which it has 30). The MTR-5B has 6 switches. Specific frequencies within each band are tuned to as described in the chart above with the MTR having the added ability of going directly to a frequency keyed in via the CW paddles. This makes large QSY's fast and easy. The frequency to which the MTR is tuned (regardless of method used) is annunciated via Morse - a handy feature in a dark tent.
The MTR contains 3 63-character CW memories. The HB-1B has a "CQ calling" function which calls CQ and then sends the "de callsign" of the operator.
The HB-1B also has an attenuator and three selectable break-in delays, one of which is QSK. The MTR is full QSK all the time and I personally would never wany anything other than full QSK with any small QRP rig.
Either rig would seem to be a good choice for the CW op wanting a very portable rig. I really like the Direct Freq Entry capability of my MTR-3B (that the MTR-5B also has) but the SW receive capability of the HB-1B would also be fun to have on a camping trip. And, although the MTR will tune to SW freqs, it is optimized for CW and lacks the bandwidth to listen to AM stations.
Finally, the HB-1B is slightly larger and almost twice the weight of the MTR-5B.
VibroPlex sells the HB-1B - LNR Precision will be selling the MTR-5B.