Hi elrock
Agree with Robert.
Pellets also require commiting to a minor operation every few months to have them implanted. This in itself isn't painful or a long operation. However, the problem comes if/when you decide to have kids.
With gels and patches, and even injections to a point, you can cease therapy immediately and residual levels of testosterone aside, can try for kids. With implants though, you will need to wait until your doctor or surgeon can arrange to get the implants taken out. This could be weeks, or even months down the line, and even then you'll likely have hormone floating around in your system for weeks after their withdrawal.
As for oestrogen? Well, in therapeutic amounts it's unlikely you'll suffer aramatisation to any extent, although it occasionally happens to some guys.
Not sure on the facts, but I reckon pellets would be the type of therapy more likely to increase oestrogen and aromatisation symptoms. Only because the levels of testosterone introduced by pellets would probably be higher than the relatively lower doses in gels and patches.
Either way, if you're young, need therapy, and aim to father children in the near future, then your doctor may prescribe HCG while you're on therapy or soon after withdrawal from it (for trying for children). HCG should get your testes producing its own testosterone and sperm again within a short time.
As Robert says though, pellets are rarely prescribed these days and patches are becoming less favourable for gels and injections. I've used injections for 12 years now (Sustanon 250mg/2weeks) and they've changed the way I feel in so, so many good ways.
Craig