Gel pros: No needle, ease of use, less risk of high hematocrit

Gel cons: Risk of transfer to others in the household, daily application needed,

Injection pros: Less frequent doses (2x/wk typical), greater increase in IGF-1 leading to greater benefits in muscle gain

Injection cons: Needles, greater likelihood of getting higher hematocrit (though still risk is only 3-5%)

To answer your question regarding prostate cancer, I will quote from this study: (key: TTh = testosterone therapy; CaP = Cancer of the prostate; PSA = prostate specific antigen)

One significant concern with TTh is potential for the development or progression of CaP, given the androgen-responsive nature of prostate tissue. In our cohort, we found no significant increases in PSA with any T formulation, irrespective of whether treated men had a history of CaP. The apparent lack of influence of TTh on PSA levels is consistent with several prospective studies that failed to demonstrate a significant increase in PSA [1],[3],[6]. However, small increases in PSA levels in men initiating TTh during the first few months following therapy initiation have been observed [29]. Furthermore, a recent study observed overall higher PSA levels in men on T [25]. However, these increases in PSA levels are not related to the risk of CaP, and support the stimulation of prostate tissue containing unsaturated androgen receptors, as described in the prostate saturation theory proposed by Morgentaler [26]. Across numerous studies, the incidence of CaP in men on TTh is low (1.1–2.3%), similar to the prevalence in the general population 1,3–6,27,28,30. In the setting of hypogonadal men with a history of CaP treated with TTh, no definitive data supporting a detrimental effect of TTh exist. In hypogonadal men on TTh with CaP treated using radical prostatectomy or radiation, small increases in PSA have been observed, with no increases in CaP recurrence rates over those seen in eugonadal men 31–34. Similar results have been observed in men with CaP on active surveillance [35]. Overall, however, these studies have been limited by their retrospective nature and small sample sizes, precluding definitive conclusions.

So there is no evience that TRT of any kind (injection, gel or pellet) increases the risk of prostate cancer. In fact, some studies have shown you are more likely to get prostate cancer if you are hypogonadal than being on TRT.