No, it isn't a "no brainer" to chose betweeen a variable and an ECB +120bps tracker (beyond the fact that a 120 bps tracker spread does seem high compared to what many lenders will offer).
Currently the spread between LIBOR and the ECB Refi rate is something above 90bps (it was when I last looked, it fluctuates). That 90bps is what is driving the 5.5% variable rate you currently pay.
Should money markets settle as some point in the next 30 years (a pretty good bet), that margin ECB-LIBOR spread will disappear again to next to nothing and the variable rate could fall below the tracker rate. ALthough it doesn't look very likely at the moment, that could (a big could) happen over the next 12 months.
But, if you have completely flexible contracts (no exit penalties), I find the best bet is just to hop from one product to the next, deciding at any particular time whether the transaction costs (solicitors fees etc.) and hassle make it worth while. Generally this might require some view on the potential period of amortisation of any transaction costs - e.g. At the interest saving you are considering (0.3% pa of capital outstanding in this case), how long would it take to recoup the transaction costs; is it likely the savings will persist that long?)