I think you'll be very happy with either, because most (if not all) of the Credit Unions in Washington share ATM's and some even share branches. I am a member at both BECU and Seattle Metropolitan CU.
I LOVE BECU. Been happily banking with them since college (so, gosh, more than a decade). Their online system is great, offering free bill pay and lots of info. They have many many ATMs (and you can use other CU ATM if need be). There are the "neighborhood centers" around, where you can ask questions or deal with loans. But if you need teller services, you pretty much have to drive to Everett or Tukwila. This hasn't been a huge issue for me, but about once a year I find myself needing to leave work a little bit early to go up to one of those two real branches. But generally everything can be handled through the ATM or online (or over the phone). And they're updating many of their ATM's to the kind that scan your check and cash, so you have a photo/accounting on your receipt (makes me more confident in doing a cash deposit).
I think the biggest choice is going to be how much teller interaction you want, and your geographical relation to their branches. My mom is only with SMCU (not BECU) because she wants to walk into a bank, hand over cash/checks for deposit to a real person, etc. So, if that is important to you, you'll want to choose the credit union that has a teller branch nearest you (the BECU neighborhood centers do not have tellers/safes, etc. They can answer questions and do paperwork, but not handle money). But if you're more interested in digital options, online bill pay, etc, I'd encourage you to investigate both CU websites to see which you like better. BECU is larger than SECU. This gives them more power/ability to upgrade more quickly, it feels to me. And if you sign up for e-statements and use online bill pay once (you don't have to do it each month, just one time) they offer a bonus higher interest rate on the first $500 in your checking and savings (and you qualify for a higher rate on your money market acct, too).
I've also been very happy with SMCU. I don't do my checking through them (so i haven't used their online system) but their credit card and savings services have been lovely. And it is kind of nice to actually walk into a bank sometimes, be greeted by employees who know your name, etc. Generally, it's fine banking with BECU only through ATMs or online. But last year an ATM malfunctioned and wouldn't release my debit card. While I was able to call the bank the next day and have them send me a new card express shipping, it was a LONG 3 days. Not being able to make a deposit or get cash suddenly became an issue, of course. But it was a freak thing and I survived it. *smile*