Hi Nikki
I would definetely try and get them into the ground rather than large pots as rhubarb really can grow huge.  We planted two or three crowns about 4 years ago into really hard clay soil, they're growing really well and give us enough rhubarb to freeze down for the whole year.
I can sypathise having the clay soil, when we first moved here it was so bad Dave resorted to using a boulster and chisel to plant a clematis.  Found out not only were we dealing with clay but the back garden had been used as a lorry park so ton upon ton of rubble.  Nearly 6 years on the ground is improving with the help of tonnes of compost, sharp sand etc.   
 
