Best sippy (for 4-6mo, maybe younger...?) we found were the colourful Heinz ones without the handles. They also have a measure on the side.
Izka ~ I was a full-time pumper, too. Just stopped at 15mo.
Best teat and bottle we found for bf and bottling was the Medela Calma. In fact, there's a picture of the teat in the Medela ad, above right. It is the only one on the market actually, truly, really made for bf and bottling (the reat just say they mimic the breast. The Calma is actually made by a company that specializes in pumps and encourages bf'ing), and it is a miracle of a thing. It has a couple of valves to allow bub to suck quite hard, as they do at the boob. It requires them to work for it in same way bf'ing does, with the added bonus that the flow is 'self adjusting' and allows bubs to take the bottle as slowly or as quickly as they like. Our boy used to get frustrated that he couldn't get enough out of usual teats, and with this one he could down a bottle in 5mins flat. It is longer and much more 'boob shaped' than any others (the 'closer to nature' ones, that are quite boob-shaped, tended to collapse for us during feeding ~ useless).
The Calma teat is, quite simply, just the job if you are swapping between boob and breast. They are pricey ~ about $18 or $22 with a bottle. Cheapest at discount pharmacies like Chemist Warehouse.
I know a little of what you are going through ~ my boy atarted to refuse the boob though, not the bottle. It can be frustrating, and it's easy to give in rather than see an unhappy baby. Must be really hard if you have to work or other reasons you need to bottle feed. Even harder if you are a full-time pumper and bubs is refusing the bottle completely. Keep an eye on his weight and hydration levels.
I hope you find a solution...Good Luck.