Short answer: yes, you want both. They protect against different things and most receiving mail servers (Gmail and Outlook especially) expect to see both pass, or at least one plus a matching DMARC alignment. DKIM proves the message content wasn't altered in transit; SPF tells receivers which servers/IPs are authorized to send mail claiming to be from your domain. Without SPF, someone could spoof your domain and send mail that still passes DKIM if they control their own DKIM key — SPF closes that gap. Add a TXT record on your root domain like:
v=spf1 include:_spf.subsimail.com ~all
and you'll be covered on both fronts.