As you can see, they are similar. Both have advance ground into the profile; 3* on the intake/exhaust on the 21 and 1.8* on the intake and 2/3* on the exhaust on the 203. For a cam only upgrade on a TC88, the 21 woudl be my choice. The earlier intake close is going to boost corrected compression which will make more low end torque. If the bike is a dyan or a softail, I mignt go for the 203 just to save money but the 21 is still a better choice for your situaation IMHO.
I/O I/C Duration Olap LSA Lift
SE-203 18/36 42/17 234/239 35 100.8 0.510/0.483
TW21N 10/30 40/8 220/228 18 100 0.498/0.498