I'm not sure anything TheDhamma has said is accurate. Perhaps those are his subjective experiences. My subjective experiences are of course different. I don't find one any more lay oriented than the other. I don't find one more reliant on monastics than the other. I don't find one more inclined to help people than the other.
Some key differences? They are so different. Even in vocabulary. Many people think it's like Christianity where a simple doctrinal difference sums it up. But it's not like that in Buddhism at all. Buddhism spread to many countries and was inevitably shaped by the local cultures and evolved over time. Much like one species of animal ends up on two separate islands and over time the species diverge. Theravada is the result of Buddhism going to Sri Lanka and other countries in that region; Mahayana is the result of it going to China and Japan and then back to India.
In a nutshell... as best as I understand...
Theravada - The Buddha taught the way for an individual to attain perfect peace. He created the monastic order as a] an ideal environment to perfect this path to peace and b] as a way to preserve the teachings for future generations. Theravada monks concern themselves with preserving the teachings as best they can, both in word and deed. Lay people try to partake in the path as much as their time allows while also making merit by supporting the monastics.
Mahayana - Apparently a reaction to certain developments at a particular time and place, the idea developed that one should not just follow the Buddha's teachings but go further and emulate the Buddha himself. Thus the idea of individual liberation was devalued and the idea of becoming a Buddha to liberate everyone was created. Also, new ideas and practices were introduced (bodhisattvas helping people is one such idea) with the aim of making it easier for the lay people to participate. Some new scriptures were authored to support these new practices and aims.
It should be noted that what it means to be individually liberated differs between Theravada and Mahayana. So too what it means to be a Buddha. So when a Mahayana teacher talks of individual liberation he is in fact not talking about the same thing as when a Theravada teacher speaks of it. This makes it hard to do a straightforward comparison of the two traditions. There is even disagreement between the two on what scriptures the Buddha actually taught and which were changed or authored after his death.
For this reason, I never recommend a person try to do a doctrinal comparison but rather experience the traditions for themselves. Likely one tradition, or maybe even one teacher, will resonate with you or make sense to you or inspire you to practice. That's what's important: being inspired to practice. Then as you develop and learn and explore maybe you'll find new teachers that inspire you or new temples. And that's fine. Eventually you'll want to commit yourself to deep study and practice in one tradition so as to get the maximum benefit out of it. Looking around in the beginning is fine and good. Hoping around from one thing to the next and never settling down is just more of the very same behavior that keeps us suffering. It is the nature of the unenlightened mind to want to keep hopping.
The goal of the Path is stillness.
I hope this is helpful.