The age old question..... :D
OK, here's my thoughts. No matter how much you think about it, you are rarely 100% sure that you want another. And that is okay!! It is a big step, and scary, and there are lots of worries that come along with it.
As to ruining your relationship with DS: It won't ruin it. It will change some. But you will be surprised how much room for love and relationships your heart has. this is coming from someone who got pregnant with the second WAY before I was ready and didn't really want DD2. I had a very emotional pregnancy, and was worried I wouldn't want or like DD2, would resent her for "interrupting" our happy status quo...but the minute she came out everything was right in the world. My relationship with DD1 did change....but not for the worse. It helped her to become more independent where she was clingy, it really brought out her loving and caring side, and it helped her to see the world and mommy didn't revolve around her. In the cases of my DD1, these were all good things.
Extra stress/work for a second: it doesn't really seem to add much, especially since moms are so great at multitasking. I EBF so there were no bottles to worry about. DD1 was potty trained so I didn't have to worry about that either, but a diaper is a diaper to me. I fix a little more dinner, buy a little more milk. I spend way more time shuffling clothes (Stored big stuff for DD1 into the closet, DD1s old to storage for DD2, big DD2 into the closet, old DD2 into a bucket to give away, etc.) which wears me out. Laundry doesn't really take any more time because the stuff is small. The hardest part I think is being sure each gets some alone time. I don't think I'm very good at that with DD2 because she is still little (16mo) and doesn't notice I don't think....but I do make a point to just "big girl" things with DD1.
PROS:
-amazing amounts of love
-there is nothing that warms my heart more than watchign my girls play together or hear them giggling in their room
-inhouse entertainment for the other
-appreciating the differences between my kids
CONS:
-space can be an issue for some. we still had all our baby stuff but they do have to share a room. my house is overrun with toys and stuff :)
-cars. we had to buy a new car so we had 2 cars that fit 2 carseats. which stinks because the one we had just died so now we have to buy another because the backup car doesn't sit a carseat.
-finances. see cars. it IS more expensive for two, but if you budget and plan its manageable
-daycare. I went back to work after DD2 and daycare for 2 is insane. After my 2nd raise I am just now making more than I pay in daycare.
-frustration. I find I get more frustrated with wanting them to behave....DD1 wants to act like a baby, DD2 is copy DD1 with big girl stuff.....I really have to take time out to remember they are each themselves.
-TIME. There is never enough. No matter what I want to do I feel as if I am sacrificing something. it is a struggle for me not to ALWAYS sacrifice myself. Thats an area I'm working on, and trying to figure out how to marry the time of me time and girls time.
--marriage. If there are any elephant problems in yoru marriage they are going to fester and get worse. trust me on experiencing that one currently.
Are the cons really cons? NO WAY. They are just little stumbles in the road. Even with all of my fear and hesitations and indecision and unhappiness about the second pregnancy and I wouldn't change a minute of it. There's never enough time, never enough money, never enough patience. But there is ALWAYS enough love. And you make it work and it doesn't seem like the analytical pro/con list.
As to my thoughts on having another. My decision was made for me (woops :P). However, I haven't decided that I am done with 2. If we have another we will wait another 3 years....which I'm nto sure I want one dangling there at the bottom so much younger when the other 2 are so close (2.5 yrs v. 4-5 yrs apart). What is my big decision factor is figuring out how DH and I are going to function and be happy in our marriage, as that is our biggest hurdle right now. I also want to make the decision together, as a choice, not as a woops. When we can do that, we will be ready whatever that decision is.
Good luck. Don't overthink it and go with your gut. You can't be wrong in this one :)