Ultimately it boils down to what subject matter you find most interesting.
The first cheilek is largely instructive in how best to achieve self growth whilst the second cheiliek goes into more detail into specific areas.
To spell it out further, albeit not completely...
The first cheilek is split into four she'arim (gates). The first provides a good overview into everyday ruchniyus (spirituality) exploring concepts such as the learning of gemara, the power of the chevrah and yomim tovim to name a few. The second sha'ar explores a combination of how to succeed in Torah study (with a specific focus on Mussar) and particular middos (attributes) with a strong focus on Emunah (faith). The third gate looks at individualised avodas Hashem (service of G-d) and the application of different knowledge bases. Finally, the last gate focuses less on the individual side and more on the klal / the impact on the many. Thus it looks at concepts such as chinuch, klal yisroel, hashgosho (Divine providence) etc.
The second cheilek is double the size of the first with the areas of study perhaps less broad. The first sha'ar is split into two - the framework of Man (i.e. what element comprises a human incl. free choice, the two inclinations, the heart etc.) and the framework of Torah (incl. fostering a love of Torah, the secrets of Torah, the sense of fear one should have for it etc.) The second sha'ar is an in-depth analysis of Mussar with the first part dedicated to how best to learn it and the second half the different areas one can work on. The third gate looks at fundamentals including chinuch, tefillah, Shabbos and Yomim Tovim. The final gate is a thorough examination of four main topics, namely; fear, truth, bitachon and life.
I hope that helps!